Single vs. Dual Subs

Using a Dual Rel T7i subs make my soundstage become wider and deeper, comparing to one subs only. Also, the bass goes deeper and smoother. I am not a scientist, so I don't know how to explain it, I can only report what I observe. Also, placing the dual subs right next to the speakers closer to the side wall makes the soundstage wider, comparing to closer toward the center .

I ran a single powered sub (15" driver) for awhile in my main listening room and then added a second.  The second made a considerable difference to the sound.

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.