OK, think I figured it out last night. There's a bit too much absorption on the right wall, and I was compensating for it by having the front wall absorption offset to the left. Problem is that the results varied depending on the amount of high frequency content, since AFAICS only the high frequencies reflect off the window behind the speakers.

Guess the next step would be to try removing the panel to the immediate right of the listening position (partially cut off in the pic above) then moving the panel on the front wall a bit to the right.

Last edited by bridgman; 09/23/14 07:07 PM.
M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8