Beth, as John(bridgman)points out, your room has an especially nasty mode at about 56Hz.This results from the floor to ceiling mode(speed of sound divided by distance back and forth, or 1130/20~56)at 56Hz plus the second order mode of both the length and width also piling up at 56Hz. Trying to reduce this by placement away from the boundaries may be a bit impractical, but you can also create a partial suckout at 56Hz by separating the M60s about 12 feet, calculated from (1130/6)x 0.3~56. This assumes that the M60s rather than a sub are mainly doing the 56Hz area. Hmmm...on second thought, have you considered moving?
