Dual vs single sub

Your subs should be capable of shaking s++t. 
How large is your room? Speakers, Amp, etc.?
B

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.