Randy, I can't quite remember what I read that prompted me to move my subs, but I tried mid-wall placement on three walls. Actually, it's more like 2/3 of the way on two walls and almost 1/2 on the other. My thinking is that if all subs are in identical placement along each wall then nothing is really being done to cancel out problem frequencies. Asymmetry seems to be best. Of course, I could be wrong.
I don't have any means to graph my room response, but I did do the SPL thing and I made a handwritten chart. My room is fairly flat in the lower frequencies. Maybe I just got lucky. Or my ears are that bad and the anomolies don't bother me. . .
