Is it better to have two matched subs or one

I am looking to complete my system.  I have one Salk 12 sub.  I am considering one more.  I enjoy classical.  Any thoughts
In general, the in-room bass smoothness improves as the number of intelligently-distributed bass sources increases.  So yes, two subs should be a worthwhile improvement over one, assuming you place them fairly far apart and at different distances from nearby room boundaries.  

Duke is certainly more qualified than I, but let me add that unlike the dedicated sub channel in home theatre sound, summing stereo bass into one channel can actually subtract some bass signals.
2 is better than one, especially if you can locate each in the optimal locations, which is usually not symmetrical.
unsound/duke, how does it subtract some bass signals?

I use single-end stereo out from an Ayre preamp to a Velodyne SMS-1 bass manager that presumably sums the channels into one balanced channel that I split between two HGS-15s subs.

There are at least two alternatives:
  • The SMS-1 has three single-ended outputs. I could take two of those to the subs. Not sure if they are summed outputs.
  • I have a spare SMS-1. I could take balanced LR from the preamp to each SMS-1 then balanced out to each sub. Set up is more complicated.
With the current setup, I’m unaware of the subs unless a pipe organ pedal note is invoked, but they do seem to make the setup sound a bit larger with large orchestrations.

Any advice-- and from you too erik -- would be appreciated.

There's a lot of unfounded info around subs, and this idea of summing bass losing info is one of them. At the 80Hz and below frequencies used for subs all bass is mono. Don't take my word for it. Hook em up yourself and listen. It simply does not matter. There is no such thing as stereo bass. Just because there's two channels doesn't mean they aren't the same. Even if they were, how would you know? We are unable to hear low bass frequencies less than a full wave. Low bass frequency wavelengths are as long or longer than the room. The sound travels clear across the room and back before we even hear it. So even if it was stereo, how would we know where its coming from? We can't. So not only is low bass not directional, we cannot hear where its coming from, and so we couldn't hear stereo bass even if it was there, which its not.
Contrary to popular belief bass can be stereo. If the bass driver from one channel is moving in the opposite direction of the other channel, when summed they can cancel out the signal between them.
@dbphd asked: "unsound/duke, how does [summing stereo bass] subtract some bass signals?"

In theory to the extent that the left and right channel bass signals are out of phase with one another, the resulting net electrical signal would be less than 100% summation. And in theory if the left and right channel bass signals are 180 degrees out-of-phase with one another, their electrical sum would be complete cancellation.

In practice, I don’t think this is a significant issue. I’ve run multiple subs in stereo and in summed mono and in dual mono and have yet to notice a difference which could be attributed to some bass signals being subtracted.

It is my understanding that true stereo south of 80 Hz or so is rare at best. 

Real (or synthetic) stereo bass can offer an enhanced sense of immersion in the acoustic space of the recording. 


" Real (or synthetic) stereo bass can offer an enhanced sense of immersion in the acoustic space of the recording." That has been my experience and why I believe most systems benefit from using subwoofers. 
@salevick - How big is your listening room? 
Is it a dedicated room or shared living room, etc.? 
Do you have a budget in mind?

Hello salevick! For classical music, one is fine. For more ooomph in pipe organ music, percussion, etc. two is obviously better. The big reason for one per channel is home theater use. Crashes, alien approaches, creatures in the bushes sneaking up on you are better since a creature menacing from the left side should not have heavy footsteps thudding from the center of the room. Many TV documentaries and nature programs (BBC stuff in particular, PBS series also) have marvelous multi-channel soundtracks. I have five systems in the home and I have 9 subwoofers. Keep Smiling!
Wild thought....CS2 footers are a new concept in isolation and using them allows the system to work closer to spec. At least that is what many users report. We find that there is no need for subs when using full range speakers, and many have removed them. 

Not sure how this would work in surround or video applications.  But in my 2 channel setup there is now thunderous bass in movies and gunshots are pretty dramatic.  
aubreybobb - By full range do you mean essentially flat to 20 Hz or below?
I've read that additional subs help with the bass "in the room" but don't most people sit in one spot? And if that spot has sufficient bass from one sub, what do additional subs add? 

Regarding "stereo" bass, what MC says about the signal below 80hz makes sense, but what if the recording mix has different amounts of signal in L & R? Then, having just one might not be getting what's supposed to be coming through the other, right? Or maybe that's just a red herring, and songs are never mixed so asymmetrically. 
Imagine your house with a stereo system in the family room to play music. You have to turn it up in the family room to hear it a little in the bedroom. It’s loud in the family room and normal in the bedroom. If you had speakers in the bedroom too connected to the same music then both rooms can be at the same loudness or normal volume. Using more than one sub does the same thing. You are blending the bass in the room which is easier with two because both can be at normal volume instead of one having to be turned up louder. Plus, The room plays havoc on bass. It’s the hardest thing to control in a system. I also have a separate theater subs for surround sound and use 2 REL subs for music. If you know someone with a sub you can borrow from try it out. You will be very happy. Then you can pull the trigger and buy the second sub. 
@hshfi Thanks. That makes sense. I have a second sub with my HT system of a different make that I can try. It's a Klipsch while my main sub is a REL. Different calibers. I will try two subs with those and see what happens.
Matched means very little with subs. Room placement is more important to low frequency distribution.
I was misinformed by REL who said their high level subs should be identical. 

bass fundamentals, and their overtones give location, imaging of jazz or rock bass player is easy and enjoyable to perceive when done right.

My speakers have 15" woofers. Think of them as a stereo pair of front firing subs. Excellent imaging of bass players, depending on the engineering of course.

Or, use a matched stereo pair of self powered subs, forward facing, adjacent to the mains to preserve imaging.

Mains: avoid ports or passive radiators, especially side/rear/bottom. Preserve direct radiation, tweeter directed to seated ear height, stereo triangle.

Toed in normally for single listener, toed in more for 2 listeners to achieve a wider center and hear both channels. Aim left speaker toward right chair, aim right speaker toward left chair. Each person is closer to one side, but gets more direct radiation from other side, works!

Preserve perfect positioning: Just mark the inside front corner of your mains, maintain that when changing toe-in amount.

Self powered woofer because if connected to sub, back to amp, it removes the amp’s need to produce low bass, AND the mains also do not try to produce low bass. Allows less powerful amp for mids and highs, and that makes trying tube amps easier.

Adjust crossover, volume, everything more easily doing it’s job.
@elliot Thanks for that helpful post. One additional element of my setup with REL is that these are high-level inputs. Possible difference there, re: Left/Right?
Subs don't need to be identical.  I have two Rythmik subs and two SVS subs.  I have them set up in stereo, but I don't think it really matters.  They're all over my room... in front of me, beside me, behind me, facing into the wall, away from me...  I have relatively small monitor speakers - the KEF Reference 1.  I have no trouble identifying where the bass player is standing on most recordings because they provide all the cues I need and build a solid soundstage with great imaging.  Listening to the alternate version of Fleetwood Mac's Rumours I could picture where John McVie was standing.  Now when I play something like Deadmau5 or Bassnectar, the bass is everywhere.
As others of said. 2 are better than 1 and they DON'T have to match.

How low do your mains cycle?
They don’t have to match, but it’s better if they do. Any difference is more likely to make localization of the subwoofers more apparent.
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The Velodyne SMS-1 bass manager shows the in-room response of my KEF Reference 1s to be pretty flat to 30 Hz before sharp roll-off, so it's no surprise the location of jazz bass players is well imaged even without the subs.  But the location of jazz bass players is also well imaged by my LS50s.

@dbphd I've had the same experience with all the speakers I've had in my room, I didn't mean to imply that the Ref 1s were doing something others couldn't, although they do image better than any other speakers I've owned.
big_greg -- I agree the Ref 1s image splendidly.  I use mine mostly for Roon via ethernet to an Ayre QX-5/20 digital hub with balanced analog to a KX-5/20 preamp and VX-5/20 amp.  For surround and movies I add a DX-5 DSD disc player, Bryston SP3 processor, and 4 LS50s.  A pair of HGS-15s with acoustic room correction extend LF to below 20 Hz.

I wish to thank everyone for your help.  My room is 14 x 24.  It is a dedicated room that opens to the house.  The three outside walls are largely glass.  I think I will purchase another sub.  I forgot to mention that I use bucket subs in the back of the room.  All suggestions are welcome.  Thanks again.  Steve