Dual vs single sub

I think Noble is on the right path here. Look at the difference between the radiation patterns of point source and linear arrays. The trick is in creating a linear array woofer. If two drivers are closer than 1/2 the wavelength of the highest frequency they are to reproduce they will act acoustically as one driver. The wavelength of 100 Hz is around 10 feet. So, if you space subwoofer drivers 5 feet apart they they will act as one driver up to 100 Hz. Say you have a 15 foot wall. Space 4 subwoofers at 5 foot intervals along that wall the outside two being right in the corners. The other two in the center should be pointed at each other with the drivers as close to the wall as you can get them. What you have created is a horizontal linear array subwoofer which minimizes room modes and reflections and takes full advantage of intersection gain. 12" drivers are way more than enough. Many will do fine with 10" drivers. 

bryantdrew,

     You haven't stated whether you have two of the larger,ported SVS PB3000 or the smaller, sealed SVSSB3000 subs.  It'd also be useful to know whether you're going to use these just for music, just for ht or a combination.
     I'm very confident that either the Audio Kinesis Swarm or Debra distributed bass array system will outperform any pair of subs of almost any size and price for music and ht.  Both are priced at $2,800 for the complete system which includes 4 subs(each sub is 4 ohms, 1'x1'x23",44 lbs, has a 10" long-throw aluminum driver and have removable port plugs so they can be run as ported or sealed.) and a 1,000 watt class AB amp/controller that powers all 4 subs and controls the volume, phase, crossover frequency and has limited e.q.
     All 4 of my AK Debra subs are configured as ported and operate in mono mode with a flat frequency response from 20-100 Hz.  I have a combination 2-ch music and 5.1 surround sound ht system using Magnepan planar magnetic speakers across the front (a pair of 6'x2' 2.7QR panels that have a deep bass limit of about 35 Hz  as mains and a CC3 center channel that has a deep bass limit of about 50 Hz.  I run all these speakers full-range with the cutoff freq.usually at 40 Hz for music and 50 Hz for ht. 
     The Debra or Swarm system provides natural, pitch-perfect, taut, articulate and dynamic bass that seems to effortlessly go as deep as the content requires and integrates seamlessly with the mid-bass to treble response from my main speakers on music.  But the system also provides these same bass qualities  along with the ability to provide deep, powerful, dynamic and accurate bass at high volumes that will shake the room when the content requires it on ht.
     I think one of these DBA systems would perform equally well in virtually any room and with any speakers.  I really don't believe it's possible for me to overstate how well this concept works and just an audition would convince almost anyone.  I'm certain that an AK Swarm, Debra or even a custom DBA using a sub amp and 4 subs of your choice will significantly outperform any system using less than 4 traditional amplified subs of any size or price; definitely in bass quality and probably in bass quantity and impact. It truly does provide sota bass response that I've never been able to equal with 1 or 2 subs of various brands in my room and system.
     It's also relatively inexpensive at $2,800 for everything but the rca interconnects and speaker cables, about the same price as 2 SVS PB3000 subs or a pair of many other high quality subs.
     I'm not a retailer and have no association with Audio Kinesis. I'm just a very pleased customer  trying to spread the word about how exceptionally well these DBA based systems actually perform.

Tim

The idea of 4 subs is the correct one. In case its not clear, the problem is standing waves and this is not easily corrected by room treatment if it can be treated at all.

But its easy to treat with 4 subs. Just do what Duke mentions above. He's a modest individual and didn't mention that he makes some of the best subs out there- the Swarm, which comes in sets of 4. Two of them are placed asymmetrically in the room and it is this placement that breaks up standing waves, allowing you to hear all the bass notes at the listening chair.

Yes four subs is the way to go. Which is why I am going that way. Frustratingly close to being done and able to offer first hand opinion! But I know it works. Or I wouldn't be doing it.