Single vs. Dual Subs

Save yourself a lot of time and effort and go directly to a Swarm.
I just set mine up two days ago and it was an epiphany.
All my bass issues were resolved immediately. Smooth, balanced, realistic low end. Now it's time to tweak things.
Very simple.  I used a MiniDSP 2x4 connected to the pre outs on my integrated, set the parameters in the software, then connected the outputs to 4 subs (two REL, one SVS and an older Sunfire). Only downside is more wires running everywhere and five more power cables. 

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.