This pic shows the room layout a bit better. This is taken from the extreme left of the masonry heater, using the chimney to stabilize the camera.



You can see all the sound-absorbing stuff on the right wall -- 4" Safe'nSound in the wall-mounted panels, 2" rigid ComfortBoard leaning against the window. I believe the panel on the front wall was just fine tuning the balance, not establishing it, but that said I don't remember if I actually tested with the panel centered between the speakers.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8