Dual vs single sub

You will have much better results adding than subtracting. The problem is none of the conventional approaches addresses the fundamental problem of resonance nodes at low bass frequencies. Two subs are better than one, but three is even better and four is better still. Beyond four continues to improve but the benefit in terms of smooth response falls off rapidly so that four is the sweet spot so to speak.

You're right about it being covered elsewhere. Unfortunately few seem to have bothered to read, or if they've read then to understand. The place to start if you're interested this review explains the what and the why better than most http://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/
Then if you want more technical http://www.gedlee.com/Papers/multiple%20subs.pdf
Then of course there are useful threads right here, although with the usual caveat of having to sift through mounds of dirt to mine the occasional gold nugget https://forum.audiogon.com/discussions/will-a-subwoofer-add-depth-and-clarity-to-my-system-or-just-b...

Or you could go straight to the gold itself
https://forum.audiogon.com/users/audiokinesis/posts

What’s your purpose in uograding, SPL, bass extension, or sound quality? 
 
Also, I know SVS has an upgrade program, but their higher end loses bang for buck due to the digital screen, remote mobile app control, room correction, etc. I mean, if you want those things, it’s great, but from raw performance, a Rythmik F18 smokes the SVS SB-16 Ultra. 
 
Dual subs give a more even response, better seat-to-seat consistency, and lowers localizability.

bryantdrew,

     I agree with millercarbon that you'll have much better results adding 3 to have the magic 4 subs in any given room that almost magically provides state of the art perceived bass response in virtually any room.  
     Of course, it only seems like magic once you actually experience the amazing transformation in bass performance in your room when you utilize 4 subs properly positioned in your room rather than 1,2 or 3 subs.  It is actually the opposite of magic since it is all based on the rigors of scientific experimentation, documented results, peer review and the experimental results have been verified independently numerous times by other scientists.
     This proven concept is called a Distributed Bass Array (DBA) system  and it consists of 4 subs strategically positioned in any given room that scientifically (not magically) provides state of the art perceived bass response throughout the entire room and not just at a specific sweet spot.
     I don't know the size of your room, bryantdrew, so I'm not suggesting you deploy 4 large SVS 3000 subs in a DBA system unless your room is huge and you're OK with weakening the structure of your home.  You could attain all the benefits of a custom DBA system in your room by just adding 2 smaller subs of your choice to the 2 larger subs you already have.  
     This concept works so well due to there being at least 4 distributed points (subs) in the room launching bass sound waves into the room.  These 4 subs are actually creating more bass modes in the room (spots where the bass is perceived as exaggerated, attenuated or even absent) but, due to the way our brain processes multiple deep bass sound waves, we perceive the bass in total as smooth and articulate.
      When I setup and listened to my Audio Kinesis Debra 4 sub DBA system in my 23x16 ft. room and initially heard the excellent bass produced, my first thoughts were that it actually did seem like magic how the bass performance in my room/system had been transformed so dramatically.  I know it's all scientifically based but experiencing what I consider sota bass in my room, on my system and on my music still seems magical to me.

Tim 
     

Room treatment. EQ.