If there's breakup that sounds almost like static distortion then it may be mistracking. But if that's the case then you should hear it on other similarly loud passages regardless of the record label. Hasten to add, I don't think that's what this is, just trying to make the point that careful listening can point you in the general direction- or if not in the right direction then at least away from the wrong direction.
You can in a similar way eliminate your electronics. Besides dynamic range records are not all recorded (pressed) at the same level. Put on one of your "loudest" sounding records, play the loudest part on it, but play it at less than full volume, and if you don't hear the "clipping" then you just ruled out the phono stage. And cartridge, phono leads, etc. Then play it at higher volume. Now if it happens its got to be the amp. Nothing else is left.
But I find this unlikely. Most of us have some kind of sense that kicks in telling us the system is too loud its getting stressed. Could be the speaker voice coils getting hot and changing resistance therefore sound, could be clipping, could be whatever. Funny thing though, more often than not the last thing we suspect is the room. Like my friend.
Remember a room is an enclosed volume. It has a resonance. Well, multiple resonances. It also has walls that are large flat surfaces that each one of which has its own fundamental resonance. Then there's the corners where the walls and ceiling meet. Its useful to think of these as a horn. Any sound originating in the corner will be amplified and reinforced by the side walls as it travels out from the corner. Therefore a small triangular acoustic panel located in the corner will be far more effective than the same size panel placed just about anywhere else.
Next most effective is along the corner where the walls meet the ceiling. Then the corners where the front, back and side walls come together.
If your electronics pass the first tests these are the logical next steps to take.
