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.
I use an Emotiva RPA-2 amp rated at 200W into 8 Ohms and 350 into 4 Ohms so it's not a problem with not enough power I don't believe. I drive my FL and FR with the RPA-2 and my center with a UPA-1 into a 4 Ohm load. I felt I was missing something using my Denon 3808 as a pre so I purchased a preamp. I was right, the preamp at about 9 O'clock is louder than the denon at half volume. The line out for the preamp is around 9 volts and the receiver is around 1.2 volts. It seems many of the avr's preouts run between 1 and 2 volts, at least that is what I was told on a different forum.
