Hi there, I'm a complete novice so take all this with a grain of salt, but for what its worth I have 4 subs arranged as follows...
Meausured they performs as follows:
Just sub 1 (SWR-1540):
Just sub 2 (SWR-1522D):
Just subs 3 & 4 (HiSAF):
This graph shows the combined result of all four subs playing (gold line) compared to side wall Subs 1 & 2 (blue line) and back wall Subs 3 & 4 (green line):
The gold line shows, in my room, that I get a flatter response overall from the side and back wall subs combined. To me, going to four subs made an audible improvement, bass seemed smoother subjectively, and I think from the sweep this is measurable.
It's also worth mentioning that despite the various distances of each sub to the MLP (center seat of back wall sofa) I have applied no delay. I have tried to add delay appropriately based on the subs relative positions but this just seems to add in big notches to the measured response in my room. I tried varying the delay up and down but coming back to zero delay seemed to get me the best result.
The big peak between 35hz and 70hz is not great however, but with a capable EQ like the DCX2496 this big peak can be pulled down somewhat to achieve something like this:
The response now varies by about 10db over the 13hz to 100hz range, where before it was more like 25db with the big 37hz peak. I made no EQ boost to achieve this.
Pulling this peak down made a really startling difference. I get heaps more tactile impact now - when wonder woman brings down the church tower you can feel it in your pants (not just cos of WW!). It also seems to reduce the very localising affect of sub 2's placement. Now the bass is kind of holographic, it just seems to come from everywhere, making the soundscape of my system seem much more spacious, a bit like being in a much larger room, like a full size cinema.
Overall then, for me, in my room/system, the back wall subs have been a success.
A final note I'd make is that being able to measure can really help. I have described how I learned how to measure in this very useful measurement microphone shoot out thread
here on AVS.
Hope this is of some help.