Ahhh--Problem solved. Adding a REL sub-bass unit...

+1 as to what Wolfe said. 

Ouch! on so many levels.

Low end is always the poor stepchild as it conflicts with parameters necessary for higher range of driver.

Phase controls adjust at single frequency. See http://ielogical.com/assets/SubTerrBlues/PhaseControl.png at http://ielogical.com/Audio/SubTerrBlues.php/

Smaller sub than main driver? Really!
Different sub models? Really!

Testing CD and no instrumentation? REALLY!

Easy == compromised.

I'd have to hear it to be convinced.

Rolling the bass out of the mains is unnecessary as the mains were likely designed to run full range, and in my experience with a few differing speakers used with my 2 RELs, that's simply a better sounding way to go. My 8" and 10" RELs match extremely well with the 12" bass speaker in my Heresy IIIs, with no phase issues (why would there when the phase is adjustable and the subs are barely in the same frequency range as the main speakers?). I don't think the avalanche of technical worries heaped on those deciding to use a sub or two is helpful really. I test my mains with a testing CD to decide where the bass becomes reticent, move the subs around until things sound great in my listening spot, and only change the sub levels from time to time to suit my mood or deal with too much or too little bass in a recording. Easy.

What do you recommend for a single capacitor that is "plug N’ Play"?

This image shows the circuit topology of hi pass

f = 1 / (2πRC)

f in Hz
R in Ohms of amplifier input
C in Farads of series cap

C = 1 / (2πfR)

This calculator will do the math. Scroll down to the calculator. Select Calculate Capacitance, enter amp input R & corner frequency. Calculation is pF. Best dielectrics are Teflon, polystyrene, polypropylene in that order and descending cost.

Parallel if you want to exact frequency, but ±1nF is close enough for most amps except those with very high input impedance. If you want to go nuts get an LCR meter and match as tight as you like, but realize the speakers are not that close, especially at the bottom!

This image from http://www.ielogical.com/Audio/#SmallestThings shows how to assemble.

You feed the full range to the sub, using its controls to roll out the highs and feed the filtered side to the mains.

When connecting for first time, disconnect mains from amplifier and turn subs all the way down. Slowly bring up subs with program playing. Should be just low end. Turn off program. Measure amp Output voltage. It should be nearly 0mV.

PM any questions.

Keep in mind
Isn't the reason you are thinking of doing this is to improve the whole system?

 If that's not what your after please don't let me keep you from having fun wondering about it.

If you agree that introducing a sub and or subs your goal is to do no harm,

Harm is for example overloading the room at room resonance blinding you to all the musics clarity you used to have.

 Harm is adjusting the sub down at the room resonance but failing to achieve a smooth overall in room response at the listening chair.

 Harm is slowing up the musics cadence pace and transient snap across the board and ending up with a lot of 90 hz or whatever frequency smearing the quality in your music.

Vandersteen Sub 3s and High Pass together with room compensation address a total solution with whole system improvement

The High pass simply connects to your main amp, unloading the main amp of its heavy lifting, but most importantly dramatically lowers the inter modulation distortion within your main speakers cabinet allowing them a new level of performance, next the11 bands per sub analog adjustments built into Sub Three amps are adjusted to fix (y o u r particular R o o ms) anomalies achieving a smoother in room bass response, improving your musics pace,transient snap, dynamics, clarity and transparency while maintaining all that's good with what you have.

 Best JohnnyR
 Audio Connection
 Vandersteen dealer 29 Years
 

@ieales What do you recommend for a single capacitor that is "plug N' Play"?
(that is minimal DIY)

@bstatmeister I received my second sub yesterday and WOW!I'm more impressed than I was expecting.The soundstage widened and feels more enveloping somehow.The performers seem more anchored in place,if that makes sense.Instead of the sax player being off to the left he is standing exactly two feet from center and five  feet back(one example).I'm hearing the effects of the beginning of cancellation of reflections and room modes.I'm lovin it!The bass is a little more articulate,I was happy with it before but the room seems more evenly pressurized now.
Right now the sub's are positioned behind and slightly to the inside,firing diagonally towards the opposite corners hooked up with their line level inputs.It sounds better in my room rather than running them at speaker level.I'll get some cheap longer cables and fool around with different placement but it really sounds good as is.I'm definitely going to add two more in the future.

Best of luck to you as you research and looking forward to your decision and results:)

2 things I am thinking about with REL:

1.)Will it go low enough for my liking? (I don't get the warm and fuzzies from only going to 28Hz on the T/9i
2.)Would I be better off, as in, better integration integration with my mains if I went the Vandersteen approach and used the low slope high-pass filters combined with the high-level speaker connection with a 2wq. Not only that, but the 2wq hits to 20Hz and is considerably heavier and is also known for being an extremely musical sub.
It seems like I might be better off with the Vandy, but I get the feeling I shouldn't discount the RELs (They are somehow calling my name, LOL)

Yep, I think people still don't quite get that

RELs are specifically designed to have the mains run through their entire frequency range. REL's manual highly advises to only use the high level speakon connector because it performs the best in that way. (I have tried REL subs using the high level connector and not using the high level connector.) This does not mean the REL doesn't provide flexibility. But it does mean it is designed to fully integrate with the mains by taking its timing cue/signal from the same amplifier was the mains, without deriving its power from the mains' amplifier--the concept being that the mains amplifier allows the REL subs and the subs to be "colored" with the same signal. The RELs allow the user to crossover from 30Hz to 120Hz, to select "0" or "180" phase and to control volume/gain. REL recommends running their subs at the lower end of the crossover range and at a  relatively low volume. I'm currently running a three REL "swarm"--it works really well.    

If you do this with the REL you are not using the high level speakon connectors, no?

I said "I like 1st order high pass on the main amplifier" - that means that you roll the low end out of the mains.

A single capacitor in series with the amp input works beautifully.

" That, my friend, is nonsense."

Well of course you can tweak your output and set the low pass frequency on the REL to get a good integration with the main, but no load is being taken off the mains (as far as the amp is concerned) using the high level connection - which I believe is how the RELs are meant to be set up. Mains will keep running per usual and the REL will fill in the low end (Am I right?) With regarding your original statement I would say if you are truly taking a load off the mains, then you will be preventing some amount of Low freq from going through the mains in the first place (like with a high-pass x-over between the pre-amp and amp). Am I wrong here?

RELs don't do it this way, they are designed to pick up where the mains drop off.

That, my friend, is nonsense.

If the sub has controls, the user chooses where to integrate. No two rooms or systems are identical. Move a complete system from one room to another and it's a whole new ball game.

Most of the time it is done terribly.

It's important to get a sub that has a similar 'signature' to the mains. Tools like REW help immensely.

@bstatmeister I honestly don't know,not having experienced what the *swarm* is claimed to do.I'm going to give a second sub a try next week and see what happens.I just wanted to point out some things for further research if you were interested.
What prof noted in his experience is interesting as well.I haven't noticed any change in timbre with my single sub in my room.I guess I'll find out soon:)My room is practically square and even with room treatments the bass sounds best when I'm in the room adjacent to it.I'm going to see if it is a positive change to further break up the reflections.I'll post the results here good or bad.

" IMO, the point of a sub it to reduce the load on the main woofer"

RELs don't do it this way, they are designed to pick up where the mains drop off. Vandersteen Subs do this, however.

I was a recording engineer. I produce live concerts. I’ve taken RTAs to venues for decades.

There is very, very little sound below 60Hz. A double bass is ≈41Hz on open E. A bass drum after ring can have some 20Hz-ish sound, but only in some halls. Piano can go ≈27Hz, but it’s hardly ever played that low. Few studio recordings have any real low end.

IMO, the point of a sub it to reduce the load on the main woofer. When the mains start heading south in an anechoic chamber, phase response and impedance are starting to go south as well. Crossover an octave higher and there is a LOT more power and control available for those frequencies because it is not being eaten up trying to drive the woofer.

Tube amps in particular benefit as transformers are not particularly good at low frequencies.

I like 1st order high pass on the main amplifier and 3rd order low pass on the subs. That puts the phase correct-ish at XOver.

If you can afford it and have the space, use two subs very close to the mains and GET THE PHASE RIGHT!!!! A 0/180° control is mostly decoration. At the very least, a continuous 0 through 180 control. It’s easy to add a ± phase switch to most subs with a DPDT switch between the driver and the internal amp.

The difference good sub integration makes is out of all proportion to the frequencies.

P.S. Martin Logan subs have tremendous flexibility and are very easy to integrate. I looked at all manufacturers earlier this year and ML offers the most bang for the buck. I'd love a pair of top of the line JL or REL, and while I could afford it, I just can't justify the cost. ML integration beats them six ways to Sunday with multi slope XO, ARC and remote control.

I come from a place of having hated subwoofers as well.

For as long as I can remember, whenever I expressed that I didn't like systems with subwoofers, the reply would be "you just haven't heard it done properly.  My set up integrates the sub properly."   Then I hear the set up and instantly hear a subwoofer.

But I had to downsize from some big floor standing speakers (Thiel 3.7s) to smaller ones (Thiel 2.7s) and I wanted to see if I could bring back some of the scale of the large speakers by trying subwoofers.

I bought a pair of JL Audio 110e subw - highly raved over even by TAS.(I have some severe restrictions in the sub I can choose - they have to fit under a projection screen and the JL subs were about the only ones small enough to do so.  Fortunately they are super high quality subs).

As they can be used from the line level from a speaker (like REL) I tried variations of that approach.  I heard some of the things you get with adding a sub - an expanded sound, more 3D imaging etc.   But the main thing I just couldn't get around was that they changed the TONE of my system.  I'm super sensitive to tone/timbre - it has to be right in order for me to want to sit and listen to a system, and my speakers/amp combo have been carefully selected to get just the tonality I find to be believable and compelling.  If I alter that, I'm playing with the most important feature of my system.

Even selecting a fairly low crossover point (again, running my speakers full range), the sound would darken overall to the degree I was not pleased.

I also bought an expensive JL audio CR-1 crossover which I've yet to try, so I'd be instead actually splitting the signal between the mains and the sub - how you are "supposed" to do subwoofers by the lights of JL audio and countless denizens of subwoofer forums.

I'll see how that goes.  I haven't gotten around to it as it's such a freaking time consuming pain in the butt.   But if I can't integrate the sub without preserving the tone/timbre of my system, I'll have to abandon the idea.

Points taken, but since I have Vandersteen 2Cs and those already go down to the low 30Hz, would I still benefit from a sub that only goes down a few hertz lower?

@bstatmeister, 

I go by sound more than specs.  I can tell you the T9i sounds musical and integrates well.  

I tune the REL by ear more than anything.  I have software and a mic to tune it but I get to ideal integration by tuning by ear.  I sometimes use a track like Neil Young's Out on the Weekend to make the kick drum sound real--not bloated.  Then I'll use some Ray Brown Super Bass stuff to get a standup bass to sound real too.  

MusicDirect and AudioAdvisor both allow in home trial periods.  I would go that route with your ears, your system and your room. 

By the way, I heard the SVS system at AXPONA.  The top end loudspeakers sounded great for the money. But and this is a HUUUUGE "But", the SVS subwoofer sounded terrible to me. It may not have been dialed in properly.  I didn't ask to play with the settings.  I can tell you my REL sounds like music.  That thing sounds or was tuned to sound big and bloated.  It was monosodium glutamate phony bass.  I'm not knocking SVS.  I'm just saying SVS stuff has amazing specs and prices and I'm not convinced their stuff is more musical than REL.   

Finally, I need tight musical integration because I use a 300B amp most of the time.  

IMHO the sub (or even better) subs don't need to plumb the depths in order to greatly improve the quality of sound in your room.Do some reading about the *swarm* that involves four subs.Millercarbon has a really interesting thread about his experience going that route.Breaking up the standing waves seems to be the more important aspect.

Does anyone who has a REL mind that pretty much all their initial lineup doesn't dig down to 20Hz? Even the S3 drops off significantly before 20Hz. Seems like a prerequisite if going to spend money on a sub....and it's the pretty much the one thing that prevents me from buying a T/9i. Convince me my fears aren't warranted....

I can mention that I have had a pair of Rel Storms that I have integrated into my two channel audio  system with floor standing dynamic speakers and planars and can also interchange with some of my stand mount speakers. Very seamless integration, trust your ears.

I added a Rel Storm MK 2 to my Quad 2905's. After a little work I have them set up where I only hear more impact.

Indeed this is a most interesting group on sub woofers views.
i am using a pair of old Gallo Nucleus loudspeakers and an old pair of 300-B stereo integrated amplifier.
The equipment is in a 13ft x 12ft room.
i listen to only classical music in particular Bach, Beethoven Chamber Music and Wagner’s operas.
The sound from the system while good, appears to lack some bass and weight.
Since I also have an old REL Strata powered sub woofer I would like to add this in the system.
i am however not too sure on the cross over frequency which best suits the system.
i would be most happy for suggestions and advice on the choice of this crossover frequency.
Thank you guys.

Look at used RELs on Ebay and Audiogon...if patient you can be rewarded with some amazing bargains.

I’m looking to add a rel to my system.  I’m hoping I can swing the 25.

Without any regard for "bothering" with calculating "phase response of their XOver, loudspeakers and subwoofer amp to integrate their sub and then measure and analyze the result to tweak and verify?," and simply using a test CD to determine how low my main speakers go, I have imaging from my system that is utterly stable and musically accurate, relative to the recording of course (there still appear to be 27 foot wide drummers here and there, but I've learned to accept them). Listeners are fooled into thinking their systems sound great all the time, as shameful as that might seem to some with a more elitist point of view, but I get it.

You took the words right out of my mouth. So much opinion spoken as fact in @ieales post. And most of it wrong.

Again, please refute with engineering facts. My focus is on music reproduction, not HT. Ported subs can go lower at the expense of linearity, phase and transient response.

I think many fail at integration because they have their sub(s) crossed over too high and with far too much gain.

Most program has no real bass. By crossing over low, almost no signal is reproduced by the sub.

Again, IMO, not enough attention is paid to phase. 80Hz is 14 feet. 90° phase error is ~3ms. Humans use the time delay between their ears for directional information. Phase error causes instruments to stroll and fatigues the listener.

In a concert hall, tympani, bass drum, bass, etc. are all localizable blindfolded and they do not stroll. In the studio, on phase coherent monitors, kick drum, electric bass, either DI or mic'd, don't stroll either.

Too many HiFi systems have horrendous low end phase which causes the aforementioned instruments to stroll or be impossible to localize on well recorded program. A poorly integrated subwoofer is a headache in waiting.

For more than 4 decades, minimum phase error has been a primary focus. For the same period, listeners always comment lifelike, accurate, precise, etc., regardless of room and hardware.

How many have bothered to calculate the phase response of their XOver, loudspeakers and subwoofer amp to integrate their sub and then measure and analyze the result to tweak and verify?

Not sure that I can contribute much to what has been posted above.  Actve floorstanders have gained a following, and IMO for good reason. I think that spending money on a quality 2 way speaker with a subwoofer or ideally 2 subwoofers is likely to be less expensive and more tunable to the room without DSP.  If I had $15k, the Vandersteen Quatro CT would be at the top of my list. But in my den (12x14x 8’’), a pair of Heresy III and a REL T9i ($3295  total new) with a crossover at 60Hz and a SET is heaven. 

Long time lurker here but I thought I would put my two cents in and echo what the OP said. We recently added a REL S-SHO 5 to our two channel system and it just makes everything sound better. In addition to improving the low end, what I had perceived as brightness in the system is now gone. The imaging seems more stable as one moves around the room and interestingly, it sounds better at low volume as well. I think it's more than a placebo effect. I don't understand the physics of why it just sounds so much better overall but couldn't be more pleased. It sounds like a whole new system. For those interested, we have Symphonic Line Legato speakers powered by Odyssey amp and preamp. Large room at 17x35 feet with vaulted ceiling. I'm sure similar results with other subs but REL was recommended by our local dealer who brought one out for an audition.
Jim

@steakster

Disinformation abounds here

You took the words right out of my mouth.  So much opinion spoken as fact in @ieales post.   And most of it wrong.   

I am a SET amp guy and, after testing many speaker and amp combinations, I firmly feel that for my setups a great stand mount monitor paired with good subs is the best way to get great sound.  Properly placed and tuned, a single driver system with subwoofer beats the pants off any full range speaker I have tested.  Again, this is using low output SET amps or SE Pentodes between .7W - 12W output.

When you think about it, subwoofers are really no more than a bi-amp solution in a box.

Some of the best Money I've ever spent was on 2 Sumiko S-10 subs. I had a Rel storm which was not big enough to take care of the 5k cu ft room adequately. It sounded good but the two sibs are much better. I considered other brands but the Rel had given me a sense of what was available with adequate power and size (12'). The Sumiko S-10 are a near equivalent to the Rel S-5 with the same specs. I highly recommend Rel.

Another +vote for REL subwoofer.  Happy Listening!

I switched from floor standers to monitors with 2 REL T-7's a few years ago and never looked back. As stated above by a few people, the flexibility of the subs to the source (a recording with low, or high bass) is the best attribute about this set up. Very happy with the sub set up.

Yup, a well-integrated sub (or two) can do that. The key word is integration. 

I think many fail at integration because they have their sub(s) crossed over too high and with far too much gain. 

My sub voyage started with a used REL Q150e that was the best 200 bucks (Ebay) I've ever spent on audio...all it needed was new grill cloth, done. Year or so later I found a REL Q108MKII also for about 200 bucks, in perfect shape, needed nothing. Made cables from Canare Star Quad, Neutrik Speakons...done. Easily adjustable, properly placed magic. No DSP needed, small se amp drives the 99db mains. Full range magic.

Very interesting comments to my OP.

I have to say that in my system (with two different sets of speakers and 3 different amps--tube, class D and class G), the system always sounds better using REL/Sumiko’s recommended way of sending the signal from the speaker terminals on the amps to the High Frequency Input on the REL or Sumiko. I’ve tried 2 other ways and the most musical was HFI and not using a crossover on the main speakers--i.e. running the mains full range.

You know it really depends on your main speakers whether or not they would benefit from a subwoofer.  As big as my main horn speakers are, they never have produced just enough bass to satisfy me even with more powerful amps,. So a subwoofer was added with excellent results. You want a sub that sounds tight and tuneful (very musical) that creates a seamless presentation.
Now in my other room I have huge but heavy custom made cabinets with 15 inch woofers, they are sealed enclosures, they are capable producing thunderous depth & bass when called upon. I would never need to add a sub to such a set-up.

@steakster 

Disinformation abounds here.

Prefer it if you can refute engineering facts. 

2. - Loudspeaker designers spend great effort creating crossovers for LF/Mid/HF bands. Mathematically it's almost impossible to mate a 12" driver to 8" or smaller driver in a vastly different box with no filter and get seamless integration. The level may be relatively flat, but the phase will be a nightmare. Until one has heard a minimum phase error system playing all 10 octaves, one has never heard bass correctly reproduced.

4. - Ported speakers have large amounts of time / phase shift, and the lower you go, the worse it gets.: "Bass reflex cabinets have relatively poor transient response, causing "smearing" or a longer resonance of the bass notes. Though the sound coming out of the port may have the same phase of that from the front surface, but it can never be at the same time, thus, the extended bass energy is really noise disguised as signal. The disguise works only when the sound is a continuous tone (one of the reason why some people prefer some particular kind of music for their audio system), but reveals itself most apparently at reproducing percussion sound."  https://en.wikipedia.org/wiki/Bass_reflex#Limitations 

As a great musician friend once said on hearing my system "Those little speakers aren't putting out all that gorgeous bass?!?!?!?!?" Informed of the sub, he stated "Every other subwoofer I've ever heard just boomed!!"

Thanks for all the interesting details, ieales. 

2. A system with powered sub should use a XOver to remove the lowest frequencies from the mains and is preferrable ot trying to ’blend’.

If the mains are capable of true 30Hz, IMO there is almost no need and even less probablity that adding subs will improve the system unless the mains are placed for WAF rather than sonics.

4. Ported subs? Never!

Disinformation abounds here.

Yes, I'm getting close to adding a center channel speaker to my setup, but will not do so without, at the same time, also adding a REL sub with the Speakon connection. 

Years back, I tried 3 or 4 center channel speakers, but when using them, I lost all the mid and deep bass presence in the center channel, compared to when I was using my front mains as a "phantom" center channel.  I tried all sorts of crossover settings in both my sub and AVR, but could not replicate the sound of deep male voice that I got out of my front L/R speakers.  So, I stuck with using the "phantom" mode.

Last year, however, I heard a home theater setup in which they were using the REL Ti series subs to extend the frequency response of some B&W 804D3 front channel speakers.  Man, it really makes a dramatic difference to get authoritative and quality bass down to 30 Hz from the front channels. 

(I always enjoy hearing what engineers hear)

Almost no one ever will. The room and electronics are vastly different. Nowadays, by the time the label gets done ’improving’, any similarity between what was mixed and what’s on the disk is quite small. Doubly so for ’remasters’ See http://ielogical.com/Audio/#ReIssues

A system without the bottom octave is a pale simulacrum and loses more that ½ the immediacy of a live performance.

IMO, most systems with subs are egregiously awful. It’s often not the fault of the sub, but the set up:
1. 0-180° Phase controls are only valid at one frequency. If the phase is not correct, the system is likely better w/o the sub. Filters change the phase response making the two controls interactive. Given that small subs have large EQ curves built into their amplifiers, the phase pot marking is next to worthless in terms of absolute phase. 180° phase switches are next useless except when used in concert with a 0-180° phase control.
2. A system with powered sub should use a XOver to remove the lowest frequencies from the mains and is preferrable ot trying to ’blend’. Doing so reduces main power requirements, driver excursion and distortion. Feeding the sub first and then the sub feeding the mains is bollox. A simple single capacitor will give a 6db/octave filter which will give a nice blend with a 18db/octave sub rolloff, provided the sub phase is inverted. Getting the phase and frequency correct is tedious. It can be calculated if one knows the amp EQ, driver response, crossover type and slope. Having a close starting point is vastly preferrable to most of the ’instructions’
3. Many systems crossover an octave too low. IMO, speakers should be crossover an octave above where they start to fail badly. In the case of small monitors like LS3/5a, Spica TC-50, that’s around 100Hz. The problem is then to find a sub with similar sonic characteristics to the mains. If the mains are capable of true 30Hz, IMO there is almost no need and even less probablity that adding subs will improve the system unless the mains are placed for WAF rather than sonics.
4. Ported subs? Never!

When properly set up, pop bass is focused and does not stroll. Tympani and bass drum become focused. The sense that one is there is out all proportion to the measured delta.
See http://ielogical.com/Audio/SubTerrBlues.php for a bit of a primer on integrating a small sub and small monitors.

I was using a pair of original Rogers LS3/5a with a Hafler SS or Marantz 8B tubes. There was no real bass without a sub. REL T2 was a gorgeous addition at a reasonable investment. I'm now using Adam Audio powered studio monitors (I always enjoy hearing what engineers hear) and though they have good bass down to 30-40 hz and sound great alone, the REL still fills out the deepest notes. REL recommends corner placement and I've always moved mine back there after experimenting. The polarity switch comes in handy for a final check of where the waves are interacting relative to your seat. And the variable crossover and volume provide endless tweaking opportunities.;-)

I was driving a pair of Dynaudio Excite x32 speakers with a Rega Elicit-R. I listen to mostly '70's rock and reggae vinyl. Sounded very good but somehow lacking. I added a pair of REL T5/i's. The RELs, once adjusted properly,  added a certain "weight" to my system, while correcting for my non-optimal listening environment. I am quite happy with them. 

Don't know how many speaker systems i 'd have gone through by now ...if i  hadn't acquired a REL.