Is there any chance you have a bunch of things "two feet from the wall", specifically the M60s ? Only guessing, but the dip around 140-150 Hz corresponds to cancellation from being "just under two feet" from a wall.

I don't really know where to start, other than to say that it's probably mains rather than sub at that frequency, and that measuring from the fronts of the mains is probably the right thing to do.

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