Recommendations for a new subwoofer?

Oh, and an afterthought: if I end up with a sub that has speaker level outputs but not a speakon connection, what would you recommend for cables? Are there speaker cables that are more amenable low frequencies, or would any good cable do the job? Thanks.

I like Rythmik a lot. They have great sound. I have a Rel HT 1003mk2, great for the price. But something about the Rythmik I just love.  Rythmik was a bit more so maybe not fair. Nemo Propaganda has reviews of both and I like his opinions. So far he has been on the money with his descriptions. Look for sales and return policies of course in case you don't like the sound.

Someone recommended Mini Micro Subwoofer cable lead - Super-Shielded - Hum-Killer on ebay (link) which I liked as well, but I've used the premium monoprice cable, monster, and haven't noticed any differences. So I don't think you need to spend a lot on cables. IMHO as always. 

I'm using two REL 212/SX subs and I couldn't be happier.

It is not looking like much of a competition. That Elac is a true sub with an excellent driver, which will dig down to 18 hz, as stated, no problem. One of the supposed competitors with a -6db down at 27hz is a ’woofer’, not exactly a ’subwoofer’.

Besides...if you have a purist rig, no measurement tools, etc, that elac came with some unique tools in its app. You put that elac in a location. Run the 2 cal sweeps with its app, one right next to the driver and one at your listening position. It will show you exactly what it measured and how it corrected for your listening position.

If it shows a huge suckout, null, etc on the app’s measurement, move the sub around and run the sweeps again, i.e., it gave you a integration tool that’s user friendly even for guys who may know not know anything about subs or room acoustics. Your chances of success could be much higher. Subs won’t magically integrate without availability and usage of the right tools...despite what the marketing said.

Ideally, you should lop your 2-way bookshelf speakers off on the low end around 100 hz or so with ’bass management’ (probably even up to 120hz with dual subs, depending on the room and not localize anything), i.e. prevent that speaker’s woofers from moving too much and pass anything below to the sub. If it moved too much, it will screw everything up at the upper octaves. In your case, it is tasked to handle everything upto 2000hz before some tweeter took over, i.e., it has a lot on its plate. So, you should be a good samaritan and lend it a helping hand. If you can eventually get your hands on something like a Yamaha R-N2000A, R-N1000A, R-N800A, etc amp with such features, it should do the job.

Hi OP

l would stick with Rel as you are there already to go with the dialling in with your pre use qualifications. No need for apps, just a good pair of ears.

Go for a T7i or T9i second hand. Both have 10” drivers, the higher the number the greater the amplifier power. No point in paying for a new X model in those sizes, there was no discernible difference to my ears. You may find a stereo pair would fit in your budget if used ones are abundant. They both have high level Speakon and low level RCA inputs. Rel developed this first and l advise you do not accept other brands that do not offer this. High level for Stereo and low level for those extra channels for home theatre…..it’s a no brainier with total flexibility. Rel may only go down to 30 Hz but l never found this a problem, my neighbours yes. Also don’t go for extra large drivers as your bass can end up boomy and bloomy. Smaller drivers are quicker and slicker.

l use two Rel T9i subs and as a guide, l bought second hand for £650 and £550 (roughly $1500) in the UK. One would be enough but two for me are best as you don’t have to drive a single sub so hard. Less chance of a bake off frying incident. You will take a risk of course with no warranty, but the choice is yours.

@cooper52 a good subwoofer cable will do the job. And no do not get a REL. They work but they're grossly priced for the performance you get

Also subwoofer doesn'magically work in a room so a bass management app is ideal. cos room modes are non negotiable. Physics can't be cheated.

Get this sub - S1512M – Power Sound Audio

Might be able to pick up a used S/510 within your budget. You’d be hard pressed to find a better sub at that size and price. 
 

And no do not get a REL. They work but they're grossly priced for the performance you get

That’s the complete opposite of my experience.

Modes, Nodes and Anti-nodes (sounds like something out of Star Trek)

A well known fact…… A stereo pair of subs actively reduce extreme room modes (and within my own experiences).  The sound waves emanate from different areas and bass dispersion is more uniform. Of course there are people with multiple subs, and stacked subs which are an altogether different room experiment.

I can heartily recommend SVS as a sub source. I have a pair of PC2000 subs that are surprisingly good and will in the right room give performance well below 20 Hz

https://youtube.com/MendItMark would have fixed it.

he informed me that one of the boards was fried

+1 REL

I checked out all the usual popular choices before making my “A” system selection.  
 

My decision tree matrix : 

(1) We have all had our fair share of bad subwoofer experiences,.There is a kind of snobbery that exists in our world of high-end audio aimed primarily at receivers, car audio, home theater and subwoofers. Poor selection of subwoofers are responsible for many people disliking them since it is the subwoofer in these situations that tends to call attention to the system and cause many of the problems. The truth of the matter is that many subwoofers have fully earned their bad reputation in 2-channel audio …Most of them sound boomy, muddy and out of control with an obnoxious bass overhang that lingers so long as to blur most of the musical information up until the next bass note is struck.

Because everything in this hobby is built to its pricepoint, it faced the tension of finding premium performance Versus budget = a $3K - max $5K audition strata .

(2) As anyone who’s fiddled with subs knows, setup is at least half the battle when it comes to getting the most out of a subwoofer 2-channel system, and I can honestly say that good set-ups are a long-time labor of love. 

(3) Best of breed build with best of breed components, and premium warranty terms.

(4) Ethereal WAF with superior visual appearances , ergo choice of all-wood finish materials.

(5);a $5000 budget ceiling price-point.

My Take: ATC  C1 Sub Mk2 active subwoofer bested all contenders and pretenders with HARBETHs in my system. 

- Superior build components, premium all wood custom finish, a full 6- year OEM warranty, and it’s a down firing unit that somehow made its unit room placement easier,

- Highly recommended for an audition.

Review: 

https://www.theabsolutesound.com/articles/atc-loudspeaker-technology-launches-c1-sub-mk2-active-subwoofer/

https://www.hifipig.com/atc-launches-new-c1-sub-mk-2-active-subwoofer/

Subs should be seen and not heard.

In other words……

1.  You can see the sub or subs (unless hidden behind furniture or out of sight)

2.  From the sub or subs, no attention sound wise should be apparent. (No sound appears to emanate from them)

If your sub-bass does not integrate and appear to be coming from your speakers and spatially within the musical performance then:-

       A. Your sub or subs are rubbish.

                               OR

       B. You have not set up correctly.

Subs should not attract musical attention to themselves. They should be fundamentally “seen, but NOT heard”

Magic happens if subs can become invisible as well. That for me truly demonstrates everything’s optimal and set up correctly.

I got some very good advice once.  If you want the best sub, buy if form a company that only makes subs.  This should shorten up your list.  Good luck and cheers. 

Parts express makes a plate amp you could bolt on the back of you old sub as long as you put an ohm meter on the speaker and make sure it is ok. Enjoy the search.

+1 for Rhythmic Subwoofers.   I demoed the REL 812 and found the Rhythmic E15HP2-SE to be far superior and less that half the price.   I bought a pair…   The Rhythmic are deep to the point of inaudibility, very articulate (you can hear and feel the string vibrations on electric bass), and exceptionally fast (they pair very well with electrostatics).   And when well setup, you really don’t “hear” them, but turn them off and the soundstage collapses.

And speaking of Rythmik (correct spelling ) Audio and electrostats: for those who want the ultimate sub/dipole combo, check out the dipole/open baffle woofer/sub that Brian Ding created in collaboration with Danny Richie of GR Research. Add that sub to your dipole loudspeakers (including any and all planars) for full range reproduction down to 20Hz. The only sub of it’s type (open baffle, dipole, servo-feedback controlled woofers) available.

Many thanks to all for your suggestions and advice. Unfortunately, in my setup, I only have room for a single sub, and my late, lamented REL S/2 filled the bill admirably. While I’m intrigued by the Rythmik subs, there seems to be an availability problem at the moment and, reading through the info on their website, they don’t recommend speaker-level connections for their subs, but right now, it’s my only option. Nevertheless, the F12 looks like it would be a logical choice for my system. Otherwise, I can see the upsides of the Elac, and I’m very tempted to go that direction despite the fact that it relies on an app. And ATC, as good as it may be, is outside my budget. 

Also, I did some online poking around about bass management but couldn’t find any useful information about how I might incorporate that into the system I currently have, and I’m in no mood to jettison my Modwright amp on that account. This is probably a totally different thread anyway, and I might post that question separately. 

Still haven’t ruled out a new REL, just have to settle on which one if I decide to go that direction. 

And a little postscript here, since it's too late to edit my original post: I mean speaker level INPUTS, not outputs. Oops. 

Speaker level inputs, not outputs. Oops

I think we all got that as a typo. 
Your observational reference about the general absence of line inputs…..

It constantly amazes me that hi level inputs are not offered as an industry standard on all decently priced (up market) subs. Why cut corners?  Maybe a deliberate marketing ploy by those manufacturers for a more customer perceived competitive retail price point versus the ones that do offer line inputs.  Hmmm

Well done Rel for taking the guess work out of it and offering line inputs AS STANDARD.

Footnote

Just to avoid any confusion, Myself and the OP are talking about high level SpeakOn (Nuetrik) speaker level line inputs.

Nuetrik connectors are regarded by professionals as the recording industries highest standard, for their robust construction and connective integrity.

@cooper52: Though Rythmik designer/owner Brian Ding prefers and recommends line-level connections, he offers his subs with a choice of plate amps that provide both high-level (power amp) and line-level (pre-amp) connections. My attitude on that subject is: Why add the distortion inherent in all power amps to the signal you send to your subwoofer amp? So that your sub suffers from the same imperfections as do your loudspeakers (the distortion of the loudspeaker’s power amp introduced into the reproductive chain)? Imo that’s a questionable approach, especially with a tube power amp.

I say, go ahead and get that T/9X. I might even got with a T/7X. I'm a fan of smaller subs for music. You can never go wrong with a REL. They seem much more musical and easy for blend with your speakers than a lot of other subs. Other subs may have more power and can go lower, but that doesn't make them sound better, at least not to me. Whatever you decide, enjoy your purchase and enjoy the opportunity to audition some new stuff. 

@cooper52: 1. "what would be a suitable replacement for that sub?" 2. "despite my previous experience with REL, is that it doesn’t rely on an app to get it dialed in to my system," 3. "Whatever sub I wind up with will have to have speaker-level outputs, because my integrated (ModWright KWI 200) doesn’t have a sub-appropriate input." 4."if I end up with a sub that has speaker level outputs but not a speakon connection, what would you recommend for cables?" 

1. As @deep_333 described, despite their use of the word subwoofer REL only manufactures Sub-Bass Systems (-6dB add on woofers).

From REL’s brochure "REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)"

If you or any of the others here recommending sub-bass systems had the opportunity to experience an in home direct comparison between an actual -3dB subwoofer and a sub-bass woofer, the ability for the subwoofer to reveal the subtle recorded extra low frequency would be undeniably apparent. 

2. Since -6dB systems so dramatically roll off low frequencies their stated low frequency is only faintly audible regardless of how many units are used. Consequently they simply do not excite a rooms standing wave bass modes requiring only a gain adjustment at the expense of the recorded low frequency presentation.

3. & 4.Your ModWright KWI 200 is equipped with a pair of RCA Preamp outputs which you would use to connect to a subwoofer input. Blue Jeans or Monoprice Cable offer affordable custom length and type of subwoofer cable.

The recording industry more appropriately uses XLR termination interconnects. Not to be confused with the pro audio public address industries use of Neutrik Speakon termination at both ends between amplification and speakers. Always consult your amplifier manufacture (not REL) regarding REL’s three wire connectivity scheme to their amplifiers.

If you decide on an actual -3dB subwoofer I suggest its imperative to first preform the subwoofer crawl to locate your rooms standing wave bass modes for to proper subwoofer location options where the subwoofer will best integrate with the room, requiring the least amount of EQ adjustments followed by any further speaker integration adjustments. 

Good luck with your search.

@mcraghead Go ahead and get that Rel T9x

At last the side of reason……

But others will say, joining the side of reason….. is not always a good reason.

@m-db 

Great advice there and overall analysis and choices to consider.

Rel recommend the bass crawl you describe. It is in their set up manual. They even suggest an actual CD track (from James Horners “Sneakers” Soundtrack) for a constant repetitive bass drum strike lasting several minutes, to find the best location.

Quite by chance l had it…….. l collect film soundtracks... Simples!

I started with a Rel Quake 20 years ago….added a second. Graduated to a T7 after one Quake failed after 15 years. So impressed with the T7 l bought a T9i. I have now sold on the T7 and bought a second T9i and now run a stereo pair.

l would have no problems recommending this company to friends or family. Rel only make subs…. It’s what they do.

I just recently discovered MJ Acoustics (UK).  I’m having a custom finish done on their top of the line sub to match my restomod Altecs, all without hearing (gulp).  But their reputation is fantastic, and my experience with respect to their customer service has been off the charts.  In your price range (a little above alas) is the Pro 50.  Might be worth checking out.

I've been through a few subs over the decades, my present pair of Rel T9x best integration of subs I've experienced. Certainly these Rels not earth/room shaking but that's not what I'm looking for. I want my subs to be invisible, simply be an extension of the main speakers, these Rels fit the bill. Now, there is an added consideration with Rels using class a/b amps vs class D, T9x use a/b. I run mine on same dedicated circuit as main amps, this 10 AWG cryo'd Romex, 20amp, Oyaide R1 receptacles, and use quality power cords, no power conditioning. The only reason I bring this up is class a/b amps far less efficient than the typical class D amp you find in subs. This means quality of AC being fed to subs will likely have greater affect on sound quality vs class D subs. For best performance you need to supply plenty of current with the REL T9x or any class a/b sub , greater impact and articulation is the result. For those who've experienced wimpy sound from class a/b Rel or any a/b sub, AC not up to need. Keeping high level cable length short as possible helps, and the quality of that cable makes a difference. Using twisted multiple runs of Duelund tin plated copper litz wire to equal 13AWG, this so perfect haven't experimented with others, I have VH Audio Airlock, a solid core wire ready to go if desired. This just to point out the high level cable used can make a difference. And all this not to say Rel a/b won't work without all this tuning, plenty using sans this. I'm just pointing out these Rels have potential some may not be extracting.

"Room shaking" is a wrong accusation received by low extension subs too many times. High fidelity infrasonic bass extension for music has nothing to do with room shaking. Perhaps, the ’room shaking’ notion comes from the extreme hometheater dudes who like to show off their platoon of 18inch ported subs hitting 120db+ every minute or something.

On the other hand, when hifi subs get down to infrasonic 10 hz levels like a couple of Rythmik F18s in one of my rooms (for example), you will experience/’feel’ something in music tracks (tracks you may have heard a thousand times) that otherwise will be fully denied to you. It’s hard to articulate color to the blind, but, once you hear/feel it integrated correctly, you won’t be able to unhear it/go back. There is always a volume knob that can be turned counterclockwise on infrasonic subs to prevent ’room shaking’ and continue to have a high fidelity experience. In fact and in most cases, it is phase integration errors that will make the room shake, and yet you can’t experience anything. If done right, one can be a hifi bass enthusiast without shaking any room, even if the sub got down to 10 hz.

On the same note, 10 hz to 20 hz is an entire 1 octave, 20 hz to 40 hz is another entire octave as far as human listening goes. For reference, 5000 to 10000 hz is also 1 octave. Perceptively, the 2 lowest octaves are also the foundation that upper octaves build up from. Why is some guy obsessed with a measly 5 or 10 hz more extension on the low end, one might think? that’s the reason...If a 35, 40 hz ’woofer’ posing as a ’subwoofer’ (missing a couple of octaves) is good enough, one doesn’t need to spend thousands of dollars on a true ’subwoofer’. A Verafi Caldera 10 priced at 200 dollars these days (a well designed musical ’woofer’) should do the job really well. OP could save himself a lot of cash.

Rythmiks are practical pricewise/sizewise for the performance they provide for most guys. On the other hand, here’s an extreme example..the sub got down to 3hz for 120k...but can it provide a hifi experience without shaking rooms? sure it can.

https://www.youtube.com/watch?v=8QOuobNCigU

Certainly these Rels not earth/room shaking but that’s not what I’m looking for. I want my subs to be invisible, simply be an extension of the main speakers, hese Rels fit the bill. Now, there is an added consideration with Rels using class a/b amps vs class D, T9x use a/b. I run mine on same dedicated circuit as main amps, this 10 AWG cryo’d Romex, 20amp, Oyaide R1 receptacles, and use quality power cords, no power conditioning. The only reason I bring this up is class a/b amps far less efficient than the typical class D amp you find in subs. This means quality of AC being fed to subs will likely have greater affect on sound quality vs class D subs. For best performance you need to supply plenty of current with the REL T9x or any class a/b sub , greater impact and articulation is the result. For those who’ve experienced wimpy sound from class a/b Rel or any a/b sub, AC not up to need. Keeping high level cable length short as possible helps, and the quality of that cable makes a difference. Using twisted multiple runs of Duelund tin plated copper litz wire to equal 13AWG, this so perfect haven’t experimented with others, I have VH Audio Airlock, a solid core wire ready to go if desired. This just to point out the high level cable used can make a difference. And all this not to say Rel a/b won’t work without all this tuning, plenty using sans this. I’m just pointing out these Rels have potential some may not be extracting.

Hello Cooper52,

I recommend a subwoofer and owner/engineer of same with whom I'm very familiar. Check out Hsu Subwoofers, https://hsuresearch.com

Dr. Hsu or his chief assistant Kevin answer the phones and will answer any questions about subs you may have.  They love to talk all things sub. I'm running two Hsu (pronounced "shoe") subs for my music and my surround rigs and love them.  Dr. Hsu's top line and newest model, the TN1 is the bomb.  I look for bass to be deep, tight and right.  Hsu hits those three on the money.   BTW that model easily fits well into your budget.  Dr. Hsu's facility is located about 20 minutes from me in Anaheim, Ca.  I pick mine up directly from his shop but all his subs, even local pick-up are securely packed in super heavy duty double reinforced triple wall cartons with metal reinforced corners.Highly recommend.  Enjoy the music.

OP here. Wow, this has turned into quite an education. Much to consider here. I’m especially grateful to deep_333 for the detailed explanation of low frequencies. Despite being a (mostly retired now) professional musician, I hadn’t really paid attention to the octaves issue (i.e. it’s a full octave from 10 to 20 Hz, and another full octave between 20 and 40 Hz, etc.) even though it’s something I’ve known since back in elementary school. 

And also, acknowledgement to m-db for pointing out that my Modwright integrated has pre-outs, which simply escaped my notice up to now. (Doh!). I actually tested it out this morning with my headphone amp just to see if it was functional, and yes indeed, the pre-outs work just fine. So that opens up the field quite a bit.

Rattling the windows and shaking the floor is definitely NOT what I’m looking for in a sub. That’s why a 10" driver is pretty much all I think I’ll need. Just needs to be musical and well integrated, with good "composure," meaning clean and well-defined. I suppose the words "fast" and "accurate" are probably more commonly used to describe these qualities. 

Right now I’m leaning towards the Elac, partly because of availability (Rythmik and MJ Acoustics are equally tempting but seem to be harder to come by at the moment) and partly because of the rated frequency response going down to 18 HZ. (The REL bottomed out at 24 Hz). Haven’t entirely ruled out REL yet, though. 

Still thinking...

Subwoofer drivers have become too damn small.

They should be at least 12 inches otherwise it’s artificial.

I hate controlling subwoofer via dsp via wi-Fi on your smart phone.  Leave the damn dials on the back of the sub. But they don't do that anymore do they?

@mylogic  Thank you for your kind acknowledgment. 

During my time with a new top of their line end table sized $9K beautifully hand veneered British made Studio III, the benefit from repositioning it to a crawl tested standing wave location was barely noticeable at best. 

The benefits of a -6dB sub-bass woofer are the simplicity of their controls and its ability to function well in most any room location due to the inability of its dramatically rolled off low frequency to excite the rooms bass modes. For the uninitiated subwoofer user, perfection. 

I used to feel that way....but, after running back and forth between the seat and different subs 1500 times, I seem to be ok with apps.

I hate controlling subwoofer via dsp via wi-Fi on your smart phone. Leave the damn dials on the back of the sub. But they don't do that anymore do they?

+ SVS and dual subs.  Meets your needs with excellent customer service.  You can call and talk to a person live if you have any questions when setting up your equipment.  

I have struggled with integrating subs. I tried dual RELs with my previously owned ATC 11s with some success but had none with my new Harbeth C7ES-XDs. No matter what setting I experienced a loss of transparency, increased midrange warmth and a less open soundstage. When I turn off the sub the soundstage opens up and the atmosphere and air return. I believe it’s because these subs bleed into the lower midrange, warming it and moving it forward. 
 

I’ve since sold the RELs. Plus these types of  “subs” are really bass enforcement. My Harbeths have really good output to about 40-45hz. So crossing over at even at the low 30s affects the lower midrange. Subs such as the REL T Series are just bass reinforcement. And they roll off into the midrange. I’d need a sub that can handle 20-30hz and with a steep roll off to avoid muddying up the lower midrange that can also allow a low crossover below 30. . 

@jfrmusic 

“I’ve since sold the Rels”

l agree with your decision. I have said a lot of good things to endorse the brand on this thread which might appear odd. Honesty and not blind biased judgement should not be on here, but yes l actually agree with you….but only in your case.

l use 40 year old Acoustic Energy AE1 speakers on their designed pedestals. I bought some newer Acoustic Energy Reference 1 speakers thinking it was a good choice to upgrade. The two Rel T9i subs l have just would not meld with the new ones…..too much bass and my settings were so low on the subs to almost make them redundant. The problem however was not with the Rels but with the speakers.

The Regerence 1’s bass response is formidable for the size of speakers. They go down to the 40Hz area you talk about. The Rels were just filling in the last 10 Hz of their specification. They were almost off just filling in one octave of range.

I lived with the new speakers for a few weeks and realised there was just no grand spacious and beguiling sound coming forth. I switched back to the old AE 1’s and put the Rels on again…..The magic returned… large soundstage and dynamics.

l came to the conclusion that the newer Reference 1’s didn’t need any subs, they were that good with just the last octave missing. End of story… l am now still using the original 1984 Acoustic Energy AE1 mk1’s and the Rels.

l think your speakers must have had the same roll off and a bit of overhang on the bass response to make the Rels meaningless.

For me it proved the older speakers still hold their own but with bass reinforcement. The newer ones are now in my second system where the extra bass is not required.

Hope this may help explain some other aspect of sub or no sub integrations 

I've never had subs in my 2 channel systems until now, i'm waiting as I write this post on a pair of REL special carbons to be delivered.  I did look at the T9s as well as some other brands, but I landed on the REL's based on speed and the particular model to match the rest of my gear.  I'll post an update once i have them setup on ease and sound. 

@mylogic 

Yes the same conclusion I had. The Harbeths don’t really need a sub. And if I wanted to add that octave between 20 and 30 I would need a sub with a crossover  below 30 and a steep fall off that does not affect the upper bass and low midrange. 

@jfrmusic 

The only other small to mid size pedestal speaker l have owned to have had the same bass response is Mission 770 Freedom’s. The front port could blow out candles and rustled my fern plants as if King Kong was behind them.

l still have them and can’t bare to let them go as l fire them up occasionally and have a blast sans Rels.

I have had Janis, Monster, REL, SVS, and quite a few others in my lifetime, and they were always in pairs. I am now using, a single..........RSL 12S, and it is exactly what I was looking/listening for.....system is 2 channel audio.... With your budget and room size, you can get a pair. I would look into them for sure. I am not affiliated with RSL, but I am a huge fanboy. My best, MrD.

I recommend a subwoofer and owner/engineer of same with whom I'm very familiar. Check out Hsu Subwoofers, https://hsuresearch.com

I didn't know Hsu was still around. I haven't heard that name in years. They were very popular at the time.

Here's an update: based on reviews, suggestions here, availability, price, and my ability to make a deal I'm comfortable with, I've ordered an Elac Varro RS-500 which should arrive in a week or two. My plan is to connect this via my Modwright KWI 200's pre-outs, and we'll see what we get. I'll post my impressions when I've had a chance to put it through its paces. 

BTW, for those who may have missed it, I posted my question about high/low pass filtering (q.v.) separately, and that turned into an eye-opening discussion which you may find enlightening. I certainly did. And no, it's not gonna happen in my system.

Doing a bit of calculating here, just for the fun of it. Modern concert "A" has been standardized to 440 Hz (though this varies with different orchestras, some tuning a bit higher, like 441 or 442). An octave below that (220 Hz) is the top line of the bass clef, and an octave below that (110 HZ) is the bottom space of the bass clef. Halving that again to 55 Hz, we find ourselves 3 ledger lines under the bass clef, which, on the double bass, is the open 3rd string (though, for those of you who might not be familiar, the double bass is always notated an octave higher than it sounds so it can be read comfortably on a bass clef--that open A string would be the bottom space) and if we halve that to 27.5 Hz, that’s quite a bit lower than any orchestral instrument can go. The double bass bottoms out at about 41 HZ, so a subwoofer that goes down to 15 or 18 Hz is REALLY gettin’ down there. 

Just a plus on Hsu subs. I run dual Hsu's in my very modest system. KEF R3 Meta stand mounts. I'm sure my system is not optimized but going to 2 subs eliminated a vexing suck-out issue I had using just one. Satisfying thump you in the chest when called for and invisible when not.

Check out their website. Easily able to get a pair for under $2K. Also might consider the new class of small subs like KEF KC62 or 92. Although I have no personal experience with the KEF subs, they look like a good fit for your situation.

That said, go with your ears and intuition. don't be swayed by the hype and snobbery of certain brands. Lots of good choices out there,

@akg_ca +1     Just installed ATC C1 Mk2 in a casita setting (200sq.ft.) with ATC SCM 7 lll and it does the sub thing…. there, but not there.  Integrated well by ear.  Great bass texture and not physically monstrous or 100lbs.  3K in this world is not obscene.  Best of luck.

I would strongly recommend that what ever you decide on it should have Active feedback of the driver. Acceleration or position to deliver 12-15 hz responses. Im getting ( measured ) 12hz from my Velodyne 18" DD+, and two Rhythmic 15" s.

Regardless what folks say there is presence from bass lower than 20 hz. Systems with active feedback are also much Faster to follow the necessary slew rates from these instruments.

@badbruno While the OP's selection seems worthy, if your Plus were beyond repair what would you do?

Given my two 12-Plus are fourteen years old I wouldn't hesitate to get a GmbH version. Pushing $10K for an 18+ or, I'd do an overnight to LA and check this dealer out. FYI...

https://www.newaudiovideo.com/index.php?main_page=contact_us&zenid=rdsd6cct0hll8mrajedonhqdr5

I'm happy with my pair of JL Audio 10" powered subs.

Work well with my B&W 803 D3.

Here's an update: the ELAC Varro RS500 sub arrived yesterday and is now going through its paces in my system. Right out of the box, I'm kind of surprised how different this sub sounds to my previous (now deceased) REL S/2, but I'm certainly not disappointed. ELAC's app has proven extremely useful in getting the sub to integrate with my main speakers (to remind you: Ryan R-610's), though I've done just a bit of my own tweaking in terms of volume and placement. Right now I have the low-pass filter set to 55 Hz, which seems to be a pretty happy meeting place. I haven't yet delved into the app's parametric EQ utility, but hope to do so soon. The utility I've only played with briefly is the phase-angle adjustment. It seems perfectly happy set at 0 degrees for now.

The ELAC definitely delves deeper into the low frequencies than my REL did, but what I'm noticing immediately is that its presentation is not quite as tightly defined as REL's. However, what I am hearing seems to be a more melodic rendering of these low frequencies, so it's not necessarily a bad thing. 

Like all new speakers, this one will probably require some significant playing time before it finds its true voice, so I'll give it a few weeks and see how/if it changes and post an update then. I have it connected via line-level RCA cables. It would be very interesting indeed to see how it behaves through speaker-level input, but that will have to wait until I really feel motivated to do it, as I'd want to invest in some decent speaker cables to try it.

Once again, thanks to all of you for your opinions and information in this thread.