Why not more popular?

A couple of years ago, I got my first set of open baffle speakers. I've owned a few pairs of Magneplanars and many box speakers over the years, but my current speakers are the first true open-baffle speakers I've owned. 

I am absolutely smitten with the sound. Musical, dynamic, powerful, and an amazing deep, open, airy sound stage, with none of the weird boxy resonances or port huffing that I've heard from so many box speakers. 

What I don't understand is why there are so few speaker companies making open baffle speakers, and why are they not more popular among audiophiles?

Just got my x5's set up yesterday.  With only 12 hours of play time they sound very promising.  They have a big, expansive rich textured sound thus far.

fleschler, nice post, and kudos on the development of the rig over the years. I laughed when I heard your wife's criticism of the panels; spot on. Those are the weaknesses in comparison to other genres. I can appreciate panels, as I have reviewed a few and owned a few.

Thoughts on speakers to move to... You do realize that Legacy can make their speakers rather active or passive version for you. Read my review of the Legacy Audio Whisper and its other iterations that Legacy Audio did; I think there are three reviews of the process in total before the final speaker. My version, the DSW Clarity Edition has capability to be run fully passive, hybrid with active bass and passive mid/treble, or fully active with Wavelet. So, there are some options for you.

If you are interested in discussing with me the Aspen Acoustics Lagrange L1 (I own the preproduction model following demo, and I have already committed to ownership of the L1 production model based on the performance of the prototype. Owner's review will appear at Dagogo.com), I invite you to contact me. I reviewed the smaller Lagrange L5 MkII for Dagogo.com. You can see the L1 preproduction model that I own on the Aspen Acoustics website.

The Aspen Acoustics is what I consider a new genre, the DLT (Disproportionately Large Tweeter), and it's VERY legit. I love it as much as any genre of speaker I have ever heard, perhaps more than omni, line source or dipole speakers of comparable size. It's an amazing blend of openness and dynamic power. The L1 has LF comparable to the Legacy Audio V and Valor, not playing around. If you wish to discuss with me the speakers I have used, shown in my system, feel free. 

BTW, since you mentioned Legacy, the i.V4 Ultra Amplifier review is published. WOW, what an amp! I am loving what class D is turning into!  :)

So far I owned EP KC IIs for a couple years, and now 3.4s (with outboard XOs) for over a year. I made 2 inexpensive improvements that paid HUGE dividends to the 3.4s

* I moved the XOs off the speaker bases which vibrate like crazy
* I replaced the crap jumper wires (from speaker binding posts to the XOs) with WireWorld internal speaker cables
Because my room is volumetrically quiet large by audio standards, I also have 2 SVS powered subs
I have owned Accoustat 2 + 2s and Maggie 3.5Rs. The Maggies came in a distant 3rd

"I’ve heard Emerald and several other boxless dynamic speakers at shows and they sounded good, with weak bass, and less dynamic than I’m used to".

You’re not talking about Spatial Audio speakers, they kill what you say on the faults of other older designed OB speakers. Time has marched on.

@mijostyn You and I shared some stats in our lives.   When I brought home the Acoustat X's, my wife let me know how ugly they were as she called them coffins.  Within a year I moved up to the Acoustat 2+2s which we had for about 5 years, the Martin Logan Monolith IIIs for 8 years.  My current wife (I'm was a widower) decided that they stunk as they were dynamic shy, muddy bass, head in a vise and bright sounding in a 25' X 20' X 11'6" room.  Four of my friends and acquaintances had Legacy Focuses so I bought a pair used in 1998.  Satisfaction for my wife and I appreciated them (although I longed for the Acoustat 2+2s).  After decades of upgraded electronics (2006 + 2019), cabling (as a beta-tester) and tweaks (Hallographs/HFTs/electrical type), I have a marvelous audio system.  The one thing I want to change are the speakers for high end types which will provide a more resolving and open soundstage in my custom built listening room.  Von Schweikert, Evolution Acoustics and now another friend suggested Usher and a San Diego speaker manufacturer whose name I forgot now are in the running.  I like Maggies and Soundlabs best of the planars and stats.   I've heard Emerald and several other boxless dynamic speakers at shows and they sounded good, weak bass and less dynamic than I'm used to.  I suppose a case could be made for them.   Legacy's "newer" baffleless speakers can sound good but most come with DSP which I don't want to use.  
Great post @phillb!

The significance of your point about Clayton using drivers designed and built specifically for OB use cannot be overstated. All professional loudspeaker designers/builders know the importance of that, and proceed accordingly. I’m sure there are some DIYers who have used inappropriate drivers (those designed for use in enclosures), but we’re not taking about them.

Danny Richie of course also uses drivers made to his specs, drivers made with electrical and acoustic properties tailored specifically for OB use. These include his woofers, tweeters, and NEO3 magnetic-planar tweeter. One difference between the designs of Danny and Clayton is Danny’s use of a minimum-width front baffle, partnered with the use of side "wings" to increase the front-to-back driver separation the large baffles used by Clayton inherently provide. In spite of their different approaches, Clayton’s design talent and resulting OB loudspeakers have received high praise from Danny.

Also good is your mention of subs not blending well with OB’s. Planar loudspeaker lovers have for decades dealt with that reality. No matter how good the box sub was, it sounded "separate" from the planar loudspeaker. In the 1960’s the 24" Hartley was used by many QUAD ESL users, Mark Levinson using it in his HQD (Hartley sub, double QUAD ESL’s, Decca Ribbon Tweeter) loudspeaker. Infinity used servo-feedback sealed-enclosure subs with their IRS and RS-1b speakers, and many other attempts could be cited.

There are numerous reasons for the difficulty of achieving the desired blending---which I won’t go into here, except in one regard:

In the 1980’s, Finnish company Gradient introduced an OB/dipole sub designed and built specifically for the QUAD 63. They followed it up with an OB/dipole for the ESL (aka 57). Though compromised by sub-par design, build, and maximum SPL, it showed the way. OB sub design flourished amongst DIY loudspeaker designers/builders, including a very young Danny Richie.

Meanwhile, Harry Pearson at TAS had come up with the idea of replacing the sub towers of the Infinity IRS with the bass panels of the Magneplanar Tympani loudspeakers, two 16" X 72" panels per side. Mated with the m/t panels of the IRS (which housed line-source arrays of EMIM and EMIT planar drivers), the sound produced was the finest many people have ever heard---to this day. Finally: full-range (except for the bottom half-octave: below 30Hz) reproduction, completely planar. Unfortunately, the resulting combo required a LOT of floor space, so was impractical for most.

Along comes Danny Richie. His GR Research company was started to sell loudspeaker kits to the DIY community, with an emphasis on OB design. GR Research was offering a number of OB loudspeaker kits, along with a sealed sub. Though very much liking OB bass, he resigned himself to the fact that it’s limitations presented a challenge he was unable to resolve to his satisfaction.

But then Danny heard about a company introducing a new (patented) servo-feedback woofer (Rythmik), which just happened to also be in the State of Texas (in Austin). Danny proposed to Rythmik designer/owner Brian Ding his idea of mating Brian’s servo-feedback woofer/amplifier with a woofer designed by Danny for OB use, a pair of the servo-feedback woofers installed in an OB/dipole frame.

As I’m sure you know (or have guessed), the product came to market. If you have heard the bass produced by the Magneplanar Tympani loudspeaker, you already know what to expect from the GR Research OB/Dipole Sub, but without the latter’s missing bottom half-octave (and massive power amp requirements!). For any OB or planar loudspeaker owner who wants not just more bass, but bass that sounds like it’s being produced by their loudspeaker, it is now available.
I've owned many good speakers from the DQ10 days, and yes they could do bass if you had a big enough amp, say the Threshold 400A they could sake the room, to Dynaudio Confidence speakers, ProAc, and Quad ESL's. I decided to try the Spatial Audio M3 Sapphires which are OB speakers. Like all new speakers, one had to allow them to burn in and learn how to set them up properly. Reading many of the anti-OB speaker comments and their sound, is nowhere what I am hearing in my system. Clayton woofers are designed for OB speakers period, in the past box woofers have been used and they suffer bass roll-off. The tweeter in the M3's was selected to work with the woofers and the open baffle and covers a large range from the midrange to the highs up to 40Khz. Once burned in the bottom end is so good, with detail, speed, and texture, that one does not need a subwoofer, in fact, I tried them and they ruined the coherence of this fine speaker. They are nowhere near boring, they make music sound alive, open, and tonally right. They produce what you feed them, you change a power cord or interconnect or speaker cable you will hear the change big time, meaning coloration is very low as well as distortion. They rock out, they play Jazz, they do baroque, large play symphonies, the piano is dead-on right, brass and horns also. They are also just enjoyable to listen to and as my wife says that sounds live. After being with my rebuilt Quads 63's by Kent at Electrostatic solutions, and enjoying them for almost 9 years, I wanted a change, I heard many speakers at my dealer and none for the money could match my Quads and they were 12-20K. Thus my research and finding Clayton Shaw speakers and after speaking with Clayton decided to take the plunge. Clayton is honest and upfront, I wanted the X5's but he suggested the M3's as the best speaker for my room and distance from the speakers. He was dead right, the wrong speaker for a given room is a waste of money because they just won't work. While no speaker is perfect, and you can always wish for if it only had this or that, but overall the M3's is one of the better sounding speakers I've heard or owned, and they are a real value vs. sound quality. After Quads, it is very hard to listen to a box speaker, with the M3's I not only listen but enjoy the difference from the Quads, and like the Quads, they just disappear when listening. Unless you hear them you cannot comment on OB designs of the past, because what these speakers do, do right is sound real as the recording and alive like few speakers can, like panel and stats, they are an open window to the recording. They cost $5,000 and worth every penny and then some. 
@mijostyn: I expected to see the response in your paragraph number one, but not from you ;-) . Yes of course, planars are technically open baffle (though there have been some ESL’s made in sealed boxes). My statement was made in response to the post in this thread claiming that "Open baffle is basically a version of what Martin Logan has been making for years."

If that’s true (and it’s not---the factory-built OB’s from Spatial Audio and Emerald Physics use dynamic---cone---drivers, as do the DIY kits from GR Research---though some with a magnetic-planar NEO3 tweeter---and Siegfried Linkwitz.), you could make the same claim about planars dating much further back than Martin Logan, all the way back to the QUAD ESL, introduced in 1957. Then the KLH 9 a little later, the Magneplanar Tympani I in the early 70’s, Sound Labs and Acoustat in the 80’s, and numerous others. Why single out the Martin Logans?! Martin Logans claim to fame is the curved ESL (designed by Roger Sanders) and the ESL/dynamic woofer hybrid (or at least popularizing it).

As for baffle size and dipole cancellation, of course that cancellation cannot be completely eliminated by increasing the front-to-back driver distance alone (that’s what Infinite Baffle is for ;-), but you can lower the frequency at which cancellation commences somewhat by doing that. What Siegfried Linkwitz and Danny Richie do is use an H-frame to create the front-to-back driver separation instead of a flat baffle, as the frame provides more resonant-free sound than does a "normal" flat baffle (an un-normal one is the 2-1/4" thick one made by Ric Schultz of EVS, mentioned above) . The depth of the H-frame is limited by the "cavity" resonance inherent in each of the frames enclosed spaces (the deeper the frame, the higher the frequency at which cavity resonance becomes audible). Both Siegfried and Danny limit that depth to around 14".

To further minimize the dipole cancellation inherent in dipoles, both Siegfried and Richie incorporate the 6dB/octave shelving circuit I mentioned above (commencing at around 100Hz, I believe). Siegfried does it in his digital filters, Danny had Brian Ding of Rythmik put it in the A370 plate amp that comes with the OB Sub kit---analog, of course.

The reason full-range planars are so big is for the reason you state---their bass output is restricted by the relatively-limited (compared to dynamic woofers) ability to move air: the thin Mylar film only moves back-and-forth a tiny distance. OB/Dipole dynamic woofers are also less able to move as much air as a sealed or vented sub, so more of them need to be used. The Linkwitz and basic GR Research use a pair of woofers (the former 10", the latter 12"), but GRR also offers a 3-woofer version. Plus, you can make multiple H-frames and stack them. What Danny does at Hi-Fi shows (and in his own listening room) is use OBs at the front of the room, and a pair of sealed subs at the rear. That set-up won GR Research "Best Bass At The Show" award several years running at RMAF (as reported in show reports in TAS and elsewhere).

Clayton Shaw at Spatial Audio and most DIY’ers go a different way: using a pair of 15" drivers on a big flat baffle. When you do that, the midrange driver (if any) and/or tweeter will also be on that baffle. For better or worse: Danny Richie is particularly adamant that it is for the worse (though he holds Clayton and his speakers in high regard).

As with everything in audio, there are trade-offs made in all loudspeaker designs. You have to pick your poison, find the loudspeaker at your price-point which gives you the most of what you are looking for. OB’s are one choice, planars another (ESL, magnetic-planar, or ribbon), horns, and of course normal dynamic (cone and dome). Infinite baffle are REALLY out-of-fashion, though they may come back in when kenjit introduces his version ;-) .
I owned quite a few floor standing monkey coffins/box speakers including; Nearfield Pipedream (10K prototypes), Genesis Vs (HPs personal pair), Usher 6371s...

and before all of those, Accoustat 2+2s with upgraded servo amps
and after the monkey coffins, Magnepan 3.5Rs, I found both of these to cut off the visual flow of my awesome multipurpose room.

I replaced the 3.5Rs with Emerald Physics KCIIs, with Clarity Cap upgrades, and wired with WireWorld OCC. Great for a regular size room, but mine is ~35 x 21 x 12. I tried to incorporate my SVS Plus and Ultra cylinder powered subs, but never could get a good seamless blend.

I replaced the KCIIs with EP 3.4s (12" concentric woofer with 1" polyester dome tweeter), which sound seamless down to ~ 40hz and blend much better with the SVS subs, but when I moved them out from ~6ft from the front wall to 7.5ft that freed them to come fully alive. IMO, the 3.4s will more than satisfactorily reproduce 95% of music we all enjoy in a more normal sized room. Quite the bargain if you can find them. There are a couple bigger EPs for sale in the classifieds

My subs are ~ 10 years old, and Im sure SVS sells vastly better subs now, but even theirs are expensive. What to do? Next up for me is EP 2.8s, which uses all carbon fiber drivers; a 12" concentric, plus 2 @ 15" subs per speaker. That should take me over the top

In summation; OBs are capable of life like reproduction at a reasonable price, and cost a lot less to ship
This is the thing I found out with an OB single baffle dipole, they are HARD to get even close. They sound wonderful, there is an immersion of sound, BUT the pole front to rear on the same baffle? THEN bounce off the front wall and it’s time aligned? HOW? It can’t be.

Dipoles on separate baffles the distance from the front wall to the back of the speaker, that is how deep and how far a speaker would have to be from the front wall. I notched the front pole to match the timing of the rear pole. You can’t do it with a common baffle. How the heck does GR do it? LOL.. You can’t! The best you can hope for it to mechanically slow the front pole and position the rear pole so close to the listening position the rear report won’t sound to far out of kelter.. NO WAY it’s gonna measure right.. BUT it sound DEEP and FULL..I could never get the sound stage right. there would be placement but no depth, then depth and no placement..

I tried until I was almost STUPID trying to figure that out with 123s.. They don’t work right.. LOL plane and simple.. Wound up on prozac because of those things..

Yes I have GRs OB servo system.. 3 doubles... Good stuff, BUT I been tinkering with direct coupling bass drivers, no passive crossover.. It actually works better for me and I'm using parts express drivers and a 300.00 Behringer 2496. I have better cone control and overshoot with class d 12k Behringer amps.. Took me a while to figure that one out... Stupid passive crossover in a bass system.. The dampening cannot work.. Short heavy copper. pretty simple.. direct coupled to heavy 12-15" drivers.. BACK it comes to center.. NOT in a forward position. Centered...
bdp24, maybe I am just an old guy and you do seem very informed but in my day open baffle meant just that and was not driver type limited. ESLs and planar magnetics are open baffle speakers and yes they are dipoles but so are any dynamic driver open to the rear. 

Next, if you think you can adjust the bass response of an open baffle woofer by adjusting the size of the baffle? Lets see, the wavelength of 100 Hz is about 10 feet. Any open baffle speaker of reasonable size is going to have bass cancelation issues regardless of the baffle size. That is just lay intuition. A driver with more surface area will produce more bass not a bigger baffle. So you see what Magnepan does with it's woofer panels. It makes the woofer diaphragms larger, many times the surface area of a 15" dynamic driver. But even with large diaphragms there are serious problems that prevent the production of adequately accurate and powerful bass in a room. Sound Labs has experimented with ESL subwoofers 7 X 7 feet, 49 square feet. Roger West is now working with old 30 inch EV drivers. Now he is a madman. 

Sand infill speakers have been around since the 50's. Warfdale I think was the first. If the final thickness of the enclosure wall is say 2" making a 2" thick MDF enclosure works just as well if not better. The primary resonance of a subwoofer is not the enclosure walls flexing, it is the entire subwoofer shaking. The way to stop this is to simply use opposing drivers in a "balanced force" configuration. The forces cancel out. Kef does this with the Blade, Magico with their big subwoofers. Boxes are easy to make but they make lousy subwoofer enclosures, better than an open baffle but that is about it. The ultimate subwoofer enclosure would be a sphere. Very difficult to build and apply. Next however is a cylinder. Wall stiffness second only to a sphere and easy to apply. Just stick a driver in each end and you have an opposing "balanced force" design in an enclosure that can be way stiffer than any box. No sand required.
I am a big fan of open baffle speakers using dynamic drivers in a 2 way configuration crossing down to an enclosed subwoofer. The open crossover can be made to look like art work. I recent heard a wonderful pair using Accuton drivers in a 1.5" thick quartz baffle board. Very cool looking. Instead of a stand they were hung on chains. What a great idea.
Here is my Q have you heard a FR, single source driver? 
No, WEll you should. 
Maybe because of room limitations as they need to be placed out into the room.
I remember the DQ10 fondly, although I agree that the bass was their weakness. 

I haven't heard other full-range open baffle designs besides the ones I currently own. But I can say that the bass is definitely not a weakness in these speakers. I think the combination of drivers specifically designed for open baffle, plenty of radiating area, and servo control, really brings it home. 

But I will agree that this requires considerably more space than a conventional speaker. I have two large cabinets for each channel, and they absolutely can't be pushed up against the front wall. I'm fortunate to have a big enough dedicated room to let them shine.

From what I have heard from others, the Spatial speakers deliver excellent full-range performance in a more room (and WAF) friendly implementation. 
My initial foray into a higher end audio system included Dahlquist DQ10, purchased after hearing at my first major audio show, thought the room with them best sound of show.
Later on became enamored with Alon house sound, owned a couple models over perhaps five years. In time I began to hear an incoherence I could never solve. Loved the mids and highs; fast, open, transparent. Bass another issue; round, slow, ponderous. Mids and highs on open baffle and bass in box didn't work imo.  I tried all manner of amplifer, could never get the coherence I sought. In the end I determined only biamping, tubes on top, ss on bass would solve speaker issue, but then we have amplifier coherence issue.
At that point I gave up on open baffle, did take a while to enjoy box speakers again. And now I've gone away from box again, big time into horns now.
FYI, in a former life, NOLA was the Alon speaker line of Acarian Systems.
NOLA is Alon spelled backward.
Both are the brainchildren of Carl Marchisotto, who has had an interesting audio career.   
@tomcy6 - Clearly not just DIY. There are a few well regarded brands. But this is a tiny fraction of the number of box speaker brands, and most audio stores don't carry any. In fact, most audiophiles that I have talked to have never heard an open baffle speaker system (at least knowingly), which is why I posed the question.
There are a number of open baffle speakers sold fully assembled.  I regularly see Emerald Physics for sale on Audiogon, and Nola speakers made a big splash in the audiophile world a few years ago and they're still around.  A google search will turn up more manufacturers, Spatial Audio and Kyron Audio, for example, so open baffle is not DIY only.

Yes @jaytor, I neglected to mention Danny has developed his own version of the NEO3, has it made to his specs, and is using it in some of his models, including yours. And thanks for reminding me of that company name---Serenity Acoustics.

There are a group of guys who hang out at the Planar Speaker Asylum, some of whom have replaced the midrange driver in their Magneplanar Tympani T-IVa's with a line of 6 or 7 NEO drivers, I believe the NEO8. The Maggie midrange driver was the weakness in the fantastic T-IVa model (I have a pair), and the use of the NEO8's (in a line source arrangement) partnered with the Maggie ribbon tweeter and double 16" X 72" Tympani woofer panels (which I spoke of above) is a killer combo. I waited too long to acquire a set of the NEO8's, and am hoping they are eventually put back into production.

I had a pair of Alons that had the tweeter and midrange mount on top of the woofer, which was inside of a box. You couldn't tell because it had an interesting angled grill cover so you couldn't see there was no box holding those drivers. They had an amazingly big sweet spot and overall they were very easy to listen to. Kept them for 25 years. 

Just last year replaced them with a pair of KEF R500s that are more detailed, dynamic and have WAY more bass, tighter, deeper. Not quite as big a sweet spot, but still very good in that regard.

I don't think you should generalize about the speaker type because each one has a different design. By and large though, total OBs can benefit strongly form subwoofers, which makes the whole system into a bigger more complicated deal. 
@bdp24 - thanks for adding all the detail. Danny's OB subwoofers are also used by GT Audio Works to great effect. These systems have received considerable praise at various audio shows over the past few years. 

The company that Danny worked most closely with that used the  Bohlender Graebener drivers was Serenity Acoustics. The company that acquired BG significantly reduced availability of the drivers (they used them in their own theatrical and pro sound reinforcement systems), which forced Serenity out of business. 

The specific models that I'm familiar with used the BG NEO10 and NEO3. Danny has been able to develop his own NEO3 replacement, which he uses in a number of his products, including the NX-Oticas which I currently own. He is also trying to create a replacement for the NEO10, but so far he has not been satisfied with the results of his prototypes.

Parts Express is now sourcing a similar driver to the NEO10 (and NEO8), but this driver does not measure as well as the BG NEO10, and also doesn't play as low in frequency, so can't be used as a substitute in Danny's Serenity designs. 

The Serenity Super 7 (which used a pair of 12" servo subs, four NEO10s and one NEO3) sold for $20,000 during it's short market life (before Serenity went under) and was (is) well regarded.

The Serenity Line Force, which used 6 NEO10s and 16 NEO3s (for each channel) in a line source configuration, along with a separate subwoofer cabinet with configurations of 2 to 4 12" servo drivers, was only shown in prototype form at shows, and used a machined aluminum open-baffle cabinet. It was expected to sell for $40K to $50K. 

Danny has continued development on both these models (when the BG NEO 10 was still available through Parts Express) to adapt them to use his NEO3 driver. The Line Source cabinets were re-engineered to use a composite material instead of aluminum which has reduced the machining cost considerably (although they are still quite expensive to produce).

I have been lucky enough to acquire enough BG Neo10 drivers from Parts Express before they became unavailable to build a set of Line Forces. I hope to have them completed later this year. 
Some of the comments are regarding open baffle seem, honestly, uninformed. To compare yesterday’s OB without listening to today’s OB and saying it’s a fad is pretty funny or sad. Box speaker’s have several limitations by design which OB resolves quite nicely. Yes, they look different, but if you have an open mind, truly listen, you will be well rewarded.  I did and created this review https://youtu.be/R9VeZOdatao
Oh, and by the way:

The Magnepan "Concept" loudspeaker---the 30.7 For Condos---is basically not that different from Danny’s NEO Line Source/OB Sub design.

The Magnepan Concept has a smaller version of the 30.7’s midrange/tweeter panel, with an OB/Dipole woofer assembly in place of the 30.7’s large woofer/bass panel. That OB woofer has multiple small-diameter woofers (6.5", I believe)---six per side, iirc. And dipole-cancellation compensation, not unlike the GR Research/Rythmik.

Reportedly to sell for less than the 30.7, if it is ever put into production. For now, a pair of the GRR/Rythmik OB Subs, with a planar of your choice (ESL, magnetic-planar, ribbon), or an OB loudspeaker (one of the GR Research kits if you’re adventurous and confident, a Spatial if you’re not) will get you very close.

If size matters, the ET LFT-8b is only 13" wide and 5’ tall, much smaller than the Magnepan 3.7i. And it’s LFT magnetic-planar drivers are push-pull designs, with magnets on both sides of the Mylar diaphram for ESL-level distortion, unlike the single-ended 3.7i midrange driver. But the 3.7i does have a nice ribbon tweeter. The MG1.7i, only $300 cheaper than the LFT-8b, suffers pretty badly in comparison to the LFT-8b. I did that comparison.
For those not bored enough already ;-) :

Danny Richie was offering a very high-performance full-range OB loudspeaker when Bohlender Graebener was still making their fantastic NEO3, 8, and 10 magnetic-planar drivers. He used the Model 3 and 8 in a line source configuration, with a separate H-frame for the OB/Dipole Sub. He also did consulting work for a couple of companies who offered factory-built versions of a very similar design. I don't recall their names, but they were reported on in various show reports.

Well-known modifier Ric Schultz of EVS (Electronic Visionary Systems) took Danny's design and came up with his own variant: the same drivers, but with both the 12" servo-feedback woofers and the NEO drivers on a flat baffle with a base, comprised of three layers of MDF with Green Glue between the layers. Danny and Ric exchanged some lively comments vis-a-vis their two designs on the GR Research AudioCircle Forum.  
@jaytor, there are a few factors which pretty much answer both the major and minor questions in your post. Because I find this subject so interesting, I shall endeavor to remember to get to them all while also getting "into the weeds", as they say. Geez, I hope that doesn't sound as pretentious to ya'll as it does to me ;-) .

But first: I can't believe this needs to be said, but it apparently does. Martin Logan has not for years been making what we call open baffle loudspeakers. Not for years, nor in fact at all. Martin Logan's electrostatic loudspeakers are not open baffle loudspeakers, they are dipole planars. So are all the other "full range" ESL's (as opposed to ESL tweeters, like the RTR's in my pair of ESS TranStatics) that are being made, or ever have been. So are the magnetic-planar loudspeakers of Magnepan and Eminent Technology, and all the full range ribbon loudspeakers, past and present.

An open baffle loudspeaker is specifically defined as a (usually) dynamic driver (or drivers) mounted on a baffle, pure and simple. No sealed or ported enclosure, the driver(s) open both front and back to the enclosed room in which they reside. To be sure, some OB designs have included a dynamic woofer and either a magnetic-planar tweeter (as in some GR Research models), or a ribbon one. The point is, the drivers are not loaded by an enclosure (with technical implications. See below.)

OB's have a long history (my first loudspeakers were OB, purchased in 1968), but have remained largely unknown to most audiophiles due to a very simple and important reason: they have been almost exclusively a DIY product, not a plug-and-play one. There is a very active OB loudspeaker "underground" community, one whose members include guys like Nelson Pass. OB enthusiasts are in one way just like the original 1950's hi-fi amplifier designers, who started making their amps on the kitchen table.

Because OB's were built, not bought, there were never any OB loudspeaker dealers, no magazine reviews, no advertising, no nuthin'. And because there was no market for them, they have for years remained largely a DIY underground phenomenon. How many guys do you know who would even consider building a loudspeaker? Kenjit would, but he can't decide which of his designs is most perfect ;-) .

The word about OB's took a giant leap forward when the design genius known as Siegfried Linkwitz (a neighbor and close friend of Nelson Pass) started publishing his papers on OB design, and introduced a number of OB loudspeaker kits. The ultimate realization of his OB design was the model mentioned by another poster above---the LX521, and is very highly regarded amongst OB enthusiasts.

Danny Richie was another long-time OB enthusiast and designer, and in the 1990's started a company (GR Research) catering to the DIY loudspeaker community. His loudspeaker products were all kits, but not all OB designs. He offered sealed designs as well, of both loudspeakers and subwoofers. While admiring Linkwitzes OB designs in most regards, it was not without reservation. Danny is that perfect combination of knowledgeable designer and critical listener (as was Roger Modjeski), and in his products address every failing he heard in the work of Linkwitz.

For instance: Danny is a fanatic about resonance, and he heard a problem in the OB sub section of the Linkwitz 521. He found the 3/4" side panels would flex when confronted with deep bass, producing a sound he describes as "buzzing". His solution? Double the sub side panels to 1.5" and line them with Norez, a resonance damper he designed and has manufactured for him. He also greatly disapproves of the digital x/o inherent in the 521, but that's a subject for another time.

Danny hears resonances in just about all commercial designs, and for his sealed sub designed a "double-box" enclosure design: an inner box and an outer one, with a layer of sand between them. Whatta nut! Danny will tell you what's "wrong" with most OB's (though he expresses his approval of the designs of Clayton at Spatial. They are friends.), and why he does things his way. For instance: he finds that an OB with a large baffle (needed to lower the frequency as which the front and rear waves meet and cancel, commonly known as dipole cancellation. That frequency is distance related: the greater the distance between the front and rear of a driver, the lower the frequency at which cancellation occurs) draws attention to the baffle, preventing the creation of a deep layered sound field. So, to create the distance required between the front and rear waves, in place of a large, flat baffle, Danny uses a baffle only just large enough for the driver, and creates the needed front-to-back distance with side "'wings". To see it in pictures, watch any number of his GR Research YouTube videos.

But OB design is much more that just the baffle. A driver used in an OB application needs to posses different technical specifications and performance characteristics than does a driver used in a sealed or ported design. Danny designs drivers for both applications, and has them manufactured for him in India. He offers his 12" woofer in both OB and sealed/ported versions, each optimized for their application. His 12" woofer is identical in most aspects to the 12" woofer Rythmik uses in their F12 sub, but with a paper rather than aluminum cone. Danny prefers the lower mass and timbral tone characteristics of a paper cone, Rythmik's Brian Ding prefers the aluminum's great stiffness.

It's been a coupla years since I spoke on behalf of the remarkable OB/Dipole Servo-Feedback Sub co-created by Danny and Brian, so I hope no one minds if I do it again now ;-) . I am not alone in considering the bass produced by the Magneplanar Tympani bass panels my standard in bass reproduction, even if their maximum output is limited (Harry Pearson agreed with me). The GR Research/Rythmik OB Sub is the only one since that could compete with the Tympani. There is one fanatic who has an Eminent technology TRW-17 Rotary Woofer for deep bass (40Hz and below, flat to 1Hz at 120dB), a pair of Tympani's for bass, and a pair of Martin Logan ESL mains. Yeah, baby!

Anyway, Danny was already offering an OB sub, and when he discovered Brian Ding's new Rythmik servo-feedback woofer had an epiphany; the mating of OB and servo-feedback, to create a new standard in bass reproduction. Both men are located in Texas, and ended up putting their heads together in the project. Brian Ding says he finds the OB Sub to be too "lean", missing the weight and heft of sealed and ported subs. Danny disagrees! All I know is that the OB/Dipole Sub reproduces the sinewy timbre of an upright bass, the fat punch of my 24" Gretsch bass drum, and the lower registers of a grand piano better than anything other that a Tympani.

GR Research provides plans for both an H-frame OB structure into which the dual or triple woofers are installed, and an W-frame design (two woofers only). Danny has a woodworker making the H-frame as a flat pack kit, very simple to assemble: just wood glue and a coupla clamps required. Fantastic frame, side panels and top and bottom 1.5 inches thick, baffle 1" thick. Makes the Linkwitz look like a 90lb. weakling!

The Rythmik plate amp included in the Sub kit includes a 6dB/octave dipole cancellation compensation shelving network, which counteracts the roll off endemic to dipoles. Not just great bass, but plenty of it. It is the only sub I consider adequate to mate with planar loudspeakers. I use them with both ET LFT-8b and LFT-4 magnetic-planars, and QUAD ESL's.

What I haven't heard are the OB loudspeakers GR Research offers. Jaytor, what beer shall I bring? ;-) 
It is hard to make a speaker that is wanted and cherished by all audiophiles there are just far too many options for that to happen at this time of audio history.
@mijostyn - I think an H-frame or W-frame design works a lot better for open baffle since it keeps the baffle size very small (barely bigger than the driver). My cabinets hardly vibrate at all (less than the sealed Velodyne subs I use in my home theater) and sound very natural to me. 

Colored reproduction ...

Open baffle colored? Surely you are confused. You might not like em, but colored they ain't...........
There is no way to do an open baffle subwoofer right. Jaytor turn up the volume and put your hand on top of a subwoofer. It will be vibrating like crazy. That is called resonance or distortion. Yes, you will hear bass but it far from accurate. I am a dipole fanatic. I also started using subwoofers back in 1978 and currently build my own. I have made subwoofer similar to your using 4 12" drivers with a three inch thick baffle three feet wide. They made a lot of bass, totally inaccurate bass. They simply could not project very low frequencies. All they were at those frequencies were room shakers. I gave up on the design in three months. I couldn't even fix it with digital room control. You would have to make the baffles out of lead to keep them quiet. When you get a chance to listen to to a system with two or more JL Audio Fathom 113's you will get the picture. The best designs now use a balance force design. Kef does it with the blades. Magico with their big subs. They put a driver at opposite ends of a very stiff, heavy enclosure that operate in phase. This keeps the enclosure from vibrating and distortion levels are much lower.
Open baffle speakers for frequencies above 100 Hz can sound great if set up correctly with sound deadening on the wall behind the speaker. Room control helps a lot. As Erik suggests it would be an interesting project doing a set of open baffle line sources sort of like the old Pipe Dreams but open baffle and 7'10" tall. The hard part is the tweeters have to be closer than 1/2" or they will not function as a line source. A ribbon like the one found in Maggie's 20.7 would be ideal then a stack of 4" diamond drivers. With sealed subs and digital cross overs that could be incredible (but very expensive) 

Few things audiophiles buy are popular.
And even things that seem popular and acceptable mostly wouldn’t be so if the PIRC saw the invoice!
Hello jaytor! Open baffle speakers are harder to place properly in a room than "boxed" speakers. My big system features the Linkwitz LX521-4 speakers: open baffle, 4 way, one amp for each "way" (eleven channels total - a 5.0 setup) and it's wonderful. Try putting a rug on the wall (hanging) behind the speakers if you're having any trouble. Thick, soft and fuzzy is best. I have two other systems using open baffle (Magnepan LRS and DIY) speakers. I love them! BUT - when a person is looking at speakers, they see the raw drivers, no nice cabinets, and they see the price tags. They know (or can easily look up) the price of the raw speakers themselves and wonder "Why am I paying XX thousand dollars for drivers that cost only 20% (or less) of that?" They forget the cost of the crossovers (lots of copper, not cheap), the packaging (much harder) & shipping, and the cost of the labor to put the thing together. It may not seem like a good value. Professor Linkwitz called his best system "521" because he tried many many baffle shapes and didn't get the "right" one until the 21st of May! "Bud" Fried's (the father of the transmission line) wife complained that she could never invite people over because the living room was always full of wood boxes and sawdust from her husband's experiments. Getting anything right takes time and money and effort.

Done right, open baffle speakers (remember the Dahlquist D-10 and some early sand-filled flat panel Wharfdales?) sound wonderful just because there is no box, no enclosure colorations. Many audiophiles are men with wives; and I don't need to finish this paragraph, do I? They just don't look like furniture (the speakers, not the wives). "You're not putting that thing in MY living room!" can be heard in many homes! I am blessed with a very wondrful and tolerant bride! Keep listening!
I have open baffle subwoofers that sound fantastic in my room. They seem to have way fewer problems exciting room nodes than conventional sealed or ported box designs. Probably similar to what can be accomplished with a swarm, but without "pressurizing" the room. The bass sounds more natural than I've heard from box (sub)woofers - I think because it is creating bass more similar to the way an instrument does. 

Using Room EQ Wizard, There are a couple of very narrow nulls at the listening position that are about 8db down due to room nodes, but 1/12 octave smoothing gives me response that is within a couple db from 18Hz up. 

My sub towers aren't small (three 12" servo controlled drivers per side), but the bass performance is worth it to me. 

Non-box speakers often have qualities that I didn’t think one could find in box speakers eg a certain openness, etc.
Non-box speakers:

—open baffle dynamics (earliest Dahlquest DQ 10), spatial audio, pure audio project, tri-art etc.
—electrostatics eg Martin Logan.
—omnis eg ohm, $25k space pods, etc
—Bipoles (old mirages). 

Clayton Saw (spatial) says his business is booming. If you like electrostatics eg Martin Logan, you like them, but they have been around forever and have nothing to do with the current popularity of open baffle dynamics eg spatial audio.
Interestingly, I bought a pair of bookshelf speakers (LSA) which have wide dispersion and have some of the same qualities
Lets see, since 1978 I have owned Acoustat Model X, Acoutstat Monitor 4, Acoustat 2+2, Magnepan Tympany III, Apogee Diva, back to Acoustat 2+2 and finally Sound Labs 645-8. I have worked at Sound Components and Luskin's both in Miami, FL and have listen closely to lord knows how many "regular" loudspeakers. 

 , you are getting close. When you take the ESL concept and turn it into a line source it is like turbo charging a car. 

To do open baffle correctly you have to have a lot of loudspeaker and all drivers have to radiate equally from the front and back or you will not get appropriate cancelation at the sides. Forget about bass under 100 Hz. It will be so lumpy you won't even be able to correct it with room control. You might think this sounds OK. It does not. Subwoofers are mandatory. A line source has to go from floor all the way to the ceiling to function as a line source over it's entire frequency range. You have to fully dampen the wall behind the speakers or you will lose detail and imaging. Making a line source that meets all of these requirements  with dynamic drivers is difficult. Planar Magnetics could come close but will never achieve the performance of a well designed ESL. The distortion produced by an ESL is orders of magnitude lower than other drivers. Their diaphragm is so light that it almost matches the impedance of air. The entire diaphragm is controlled by the signal. The large surface area is capable of transferring huge amounts of acoustic energy to the air (as long as there is no bass below 100 Hz). Only horns can match this.
My guess is that my current system will have no trouble getting to 110 dB.
I'll have to wear hearing protectors to find out. At regular volumes thing like snare drum hit will slap you in the chest. 

So, what is the main difference? When I listen to music on dynamic speakers, baffled or not I know I am listening to a reproduction. When I am listening to a good live recording on big ESLs, I am there. The volume and power are the same. The speakers and the walls disappear. 
The only downside is you have to deal with the size which for many people is hard to swallow. 

I think they sound really good at volumes at or below conversation levels.

And while there can be some details that aren't as clear as other manufacturers I think the sound stage is very good.

My friend has a set and there's a difference between his system and mine and they both encourage music enjoyment.
I have to agree with @wrm57 . I also purchased a pair of well known open baffle speakers. I describe them as the best, most boring speaker I ever owned. They certainly were very detailed and had a great sound stage but just didn’t have the same warm, engaging, musical feeling of a box speaker. I own Legacy’s now.
You know, conceptually I like open baffle speakers a lot more than I like ESLs.

Ages ago I was smitten with the detail and air electrostatic loudspeakers (ESL) s could provide, timbral accuracy be damned.

Open Baffle speakers, like line arrays, have the potential to overcome all those shortcomings while being overall easier to drive than ESLs, not to mention a wider sweet spot.
Open baffle remind me of fat gym teacher. Something just not right
. @tubebuffer that is funny.
Some of Legacy Audio's more expensive speakers are partial open baffles. Legacy calls these speakers hybrid dipolar. Very well regarded sound quality.
I owned a pair of EP KCII Pro's for the better part of a year.  I knew I was not able to set them up ideally--could only place them 30" off the front wall.  Even so, they imaged wonderfully--of course, depth was limited but excellent height and width.  They also had explosive dynamics.

OB bass is very inefficient bass, due to wave cancellation.  So you need really big drivers placed well out into the room to mitigate this.  So in my experience, that tight, fast OB bass could also be described as a major lack of bass.  So that required subs.  

The deal breaker for me was a peak around 3-4khz that would rear its head on too many recordings.  So out they went, replaced by a wholly satisfying box loudspeaker.

Like many things in life, it's not the pathway you choose, but how you execute that choice.
My only experience was owning a pair of large Alon speakers where the mid and high range drivers were mounted on an open baffle.  I really tried to like those speakers (over several years and upgrades) but never got to a sound that seemed right to me (maybe a perceived lack of drive and dynamics).  I basically ended up in the same place as @tubebuffer - 
"Open baffle remind me of fat gym teacher. Something just not right."
Have dipoles, PAP’s, most likely, never going back to boxes. Like a good woman, when it hits your soul, one only listens to the music. 
Open baffle speakers are an entry drug to dipoles, which leads one to omnis......*smirk*G*

One has to 'listen outside the box', Literally....

(btw, I have and still Do own some 'boxes'....one needs to remind oneself what they left behind.....and Why...)
I listen to a pair of Tekton open baffles (Their sigma model) daily in the home theater & really enjoy them. I do not like them as well in the stereo system as my current box speakers (Wilson cubs) however.   
@audioman58, The AMT horn-loaded tweeter in the X series is a Beyma, made in Spain, great driver.
What I don’t understand is why there are so few speaker companies making open baffle speakers, and why are they not more popular among audiophiles?

Because we all hear differently, have different tastes, different rooms and different systems. Different companies have different technologies or design philosophies they want to feature.

If I could listen to every speaker out there in my room, with my electronics I would probably end up with a different pair of speakers than what I have. Different electronics too. Since I have to buy something in order to listen to it, and I buy used for that reason, my selection is pretty dramatically restricted. Most of us here are probably in the same situation. So we end up with the best that we can do, not necessarily the best possible.