[drpaulng]

No tri-corners?
Good point.

Tri-corner is the best place for exciting all room modes of a "shoe-box" listening room. A rectangular room is supposed to be well-predicted for room modes calculation (in fact it is). In order to eliminate the need of tri-corner, we might place an anti-phase sub very close to one of the stereo full-range speakers and feed to it simultaneously with music. The anti-phase effect should work out well to fight against the room modes (The sub is pre-filtered with parametric EQ. and plays only the anti-phase room mode peaks at selected frequencies, not acting normally as ordinary sub).

I propose some would-be reasons of failure here:
(1)The anti-phase adjustment might not be too easily adjusted to fully 180deg against the excited room modes however closely sided with the main speaker.

(2)The phase adjustment circuit cannot adjust the frequencies simultaneously in one phase (some of them lag behind the others).

(3)The room interaction is not predicted well. The anti-phase sub should be indivdually re-adjusted to copy the room modes of the main speaker since they are placed side by side, not overlapped in exact position.

(4)The main speakers should be full-range in order to play good stereo, not the 2.1 system. However, if it's a 2.1 system, put the anti-phase sub side by side with the 0.1 subwoofer, not the small stereo speakers.

I got this idea when I accidentally reverse the speaker cable connection to one of the front speakers. The low frequencies were attenuated very much while the upper range still seems to be OK. Obviously, the "lows" are much more "in phase" when coupled to the air while the "highs" are relatively immuned to the reversed phase. So, it is up to people who can carry out experiments with an extra sub. Sorry, I don't have an FBD at the moment.

Edit: Point 4, and some typos.