I think it is a combination of room acoustics and that the M80s don’t roll off the highs. One of the several reasons I bought he M80s was that unlike most other speakers that do roll off the highs the louder the M80s get the highs still keep up. In a potentially “bright” room this can sound quite harsh IMO.
Before installing my curtains I had a similar experiences with my M80s here in my house even with a carpeted floor. However, now with the curtains up 270 degrees around the room there is no harshness no matter how loud I push them as long as their not distorting.
Until I installed my curtain I just used the “front treble” to reduce the high end when playing them loud.
