Without hearing exactly the same thing you were, it's impossible to say.
No, we're talking about would lead to it being a room problem. We're saying that because the M80s don't roll off the highs, as the room loads up, they become over-bearing. Where as other speakers that don't have highs which can keep up with the level of the rest of the music range won't have the same effect.
The other likely candidate, is the receiver is running out of power. That will cause a sort of dynamic range compression too, and also cause the sound field to shrink. This one is slightly more likely from your description, in that you mentioned a blurriness, that may be the onset of clipping, or a gross amount of harmonic distortion.
I guess you could buy bigger amps, and see what happens, if that doesn't solve the problem, go with more absorptive room treatments (although that may dull lower listening levels). Or just don't play it that loud.
Where were you on the volume knob anyway? -9, 0, +3?
