About a year ago when I was playing around with the realtraps test tones and manually plotting my room, there were certain frequencies that appeared to have a piercing buzzing sound. Not sure if it is what your describing, but it appeared to be related to the room. I've never had an issues with music or movies after those tests.
If you swap the location of your m80's does it still happen? Axiom also asked me if I run a lot of frequency test tones for extended periods of tiime, hinting it might not be good for the drivers.
