Single vs. Dual Subs

Pray, do tell us all.
Integrated model and connections, Main speakers, room dimensions, DSP parameters, sub models & location

As of this moment.
Musical Fidelity M6si integrated amp.
Vienna Acoustics Mozart Grand SE
Audioquest Black Lab sub cables
Room 9"6" high x 12' wide x 16' long
Speakers 4' from front (12') wall
Rel T5 (2) located behind main speakers 24" from front corners
SVS SB-2000 rear left corner
Sunfire True Architectural Signature Sub behind couch. Midway along right wall.
The MinDSP has just a basic setup of all crossover points at 70hz.
I have downloaded Room EQ Wizard, purchased a MiniDSP Umik-1 microphone and will be doing an analysis when I am feeling motivated. This allows me to create a parametric EQ file to import and do a final tweak. REW will also provide me with a roadmap for adding room treatment as required.

Read Floyd Toole's book before embarking on room treatment.
Are you driving the mains full range?
What type of XO [BW,LR, BE & order] are you using?

Indeed, the multiple sub solution outlined by Toole and on which the Distributed Bass Array approach is based is a game-changer. Multiple subs distributed asymmetrically around the room effectively minimizes the problem of bass modes. Then when the subs and everything else are isolated on Townshend Podiums and Pods this virtually eliminates another major source of room mode excitation. The combination of the two is stunning. It takes a huge amount of bass traps and other room treatments to match what isolation and DBA can achieve.  

Then if on top of all that you go to Townshend F1 cables, these eliminate yet another source of harmonic resonance. 

The old-school approach of EQ and room treatment really is old school. There are much better solutions available. People are catching on. Good to see.

Miller +1on subwoofer isolation.

My subs have been supported on foam padded dollies with extra soft rubber wheels for many years now.
After a move and before a permanent system setup we had our single 18" sub sitting on some folded moving blankets. We noticed how much less the low frequency excited the structure while still providing deep bass.

When we purchased our current 12" subs ten years ago I connected the 12s and the 18" with RS-232 cable and the home theaters 15" Super Nova with L/R interconnects. Effectively creating what's now called a swarm which did indeed eliminate all the rooms bass modes.

We noticed little if any sonic advantage between Cardas Golden Reference XLR to Canare XLR.
On the other hand when I applied the four subwoofers with fourteen parameters of the subwoofers Room Optimization the improvement was stunning while adding the flexibility of remote controlled preset equalization. 

I'm very interested in the "much better solutions available" you mentioned?