I'm having similar problems with my on-wall M22's as edmond is experiencing with his M60's, although I've got furniture and a carpet. I have the Pioneer Elite VSX-21TXH avr and it has an option I just thought of that I haven't tried out yet. In the MCACC menu it's called X-Curve. It allows you to tone down the high end frequencies. (The + or - for Treble seems to only work while in stereo mode).
Here's what the manual says about X-curve:
"Most soundtracks mixed for cinema sound too bright when played back in large rooms. The X-Curve setting acts as a kind of re-equalization for home theater listening, and restores proper tonal balance of movie soundtracks."
I watched the movie "The New World" yesterday with the volume turned up a bit and the high end was making it a little uncomfortable to listen to.
