Single vs. Dual Subs

divertiti, I have a similar situation to you, relative to Sub Woofer Placement.

I solved it, which IMHO, has optimized the results, as follows.

Here's my solution:

1) Set of 12 inch, self powered 500 Watt Subs at Far Left & Right of room, set @ 70 HZ Crossover.

2) Single 15 Inch Sub Directly in Center.  This Sub Woofer is Fed its signal from a dbx 234S Electronic Crossover set @ 40 HZ Crossover Frequency, & set to SUM Left & Right Channel Inputs, which signal is in turn fed to a Classe 25 in Mono Mode, putting out an honest 1,000 Watts.  dbx 234S gets its signal directly from the second output of my pre-amp.

3)  Combined with PS Audio BHK  Signature 300 Mono Blocks, feeding a pair of Anthony Gallo Acoustic 3.0's, three way speakers.

This combination of Speakers puts out Exceptional Non Boomey Bass with Massive Slam, as the 15 Inch Sub Woofer is driving directly into one, when they are sitting in the sweet spot.

I find the "Single Very Low Bass Sub", summing both Left & Right Bass signals to be optimum for Deep Bass, while not interfering with the music that one plays.  

It is all summed at that frequency anyway. But, whatever.

“Two or more subwoofers also help in reducing low-frequency distortion and offer improved dynamics over a single subwoofer. This is a simple function of having more radiating area, plus more power on tap.”

 I suspect that we all understand that bigger drivers and more powerful amps are usually good for bass. ‘Twas ever thus.  But many of us have $, space and spouse constraints.
I’m considering getting sub(s).  My question is more like, “for the same money, are 2 10” subs likely better than 1 better (and more powerful)  sub?”   E.g., 2 REL t/9i vs 1 REL S/510 cost about the same. 
For that matter, is there a consensus (here? Lol) that REL is likely better for 2-ch than JL Audio or SVS?

All IMO:

• the most bang for the buck are the Martin-Logan x series. Complete control set, available Perfect Bass Kit make integration a doddle.
• JL Audio are very good, but pricier.
  
• REL ain't what they used to be and are more jewelry than engineering. Their web malarkey is geared to the uninformed and WAF wimps.
• Never heard or used SVS

My perspective is from fidelity and integration standpoints. If you want to be punched in the stomach [which does not happen when recording a kick drum] I'm outta my depth.

see  ieLogical SubterraneanHomesickBlues for more info and a primer on integrating small subs.