It could also be that your amp is straining at those higher volume levels, what amp are you using to drive the M60s?

A few forum members have had success with using separate amps when trying to listen at high volume levels as there amps/avr's were straining abit to reach those sound levels. The extra power cleaned up the sound for them.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70