Guilty! Three subs in fact. The two flanking the 8c's and one to the left of the MLP in the tail of my L shaped room (positioned equidistant between MLP and 8c's).
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A lot of ifs and buts here I think, such as the capabilities of the subs (freq & transient response at higher xover freq) and what you are trying to accomplish by using subs?
I use the subs because they were already there and optimised for the room, however I do believe there are some advantages to using them in my situation instead of the 8C's on their own.
-The 8c's are high passed at 120hz, which quite noticeably improves their headroom. 120hz is the limit for my flanking subs before they start to sound clumsy. I've ordered a pair of @sigbergaudio subs which are tuned flat to 200hz, so it will be interesting experimenting at say 150hz if they ever arrive.
-Each sub is running in mono (both L&R channels enabled in the DSP). This produces (in my subjective opinion) smoother bass and greater timbral accuracy compared to running them in stereo, without sacrificing stereo imaging.
-The flanking subs being on the floor and against the front wall eliminates any SBIR issues, though ditto the 8c's boundary filters.
-The third sub in the L to the left of the MLP helps smooth the response before eq is applied.
-The overall sound is bigger, more enveloping and effortless than the 8c's on their own. I suspect I loose some bass output having them two feet from the front wall and in fairness to the 8c's I haven't spent much time optimising them running solo. Having the subs outside the 8c's widens the sound stage and whilst theoretically 4x8" drivers should move as much air as two 12" drivers, the bigger drivers just seem to produce more impact.
As always, YMMV of course.
I like your thinking and system. Care to tell us more about the subwoofers integration process and particulars?