I'm not talking about amplifier clipping, but in the case of amp clipping, the piercing sound is a result of harmonics, not the output signal itself. The output signal when clipping resembles a square wave which by definition means that the clipped portion of the signal contains no dynamics.

What I am really talking about is an amp compressing the signal before reaching clipping. I'm sure we've all had the experience of playing with the volume on a receiver and up to a certain point, the volume adjustment seems very linear - turn the knob and the music gets proportionately louder. But go beyond a certain point and the volume doesn't respond in such a linear manner any more - the music doesn't get proportionately louder, it just becomes less musical and compressed. Of course, if the volume is turned increasingly higher, the amp will eventually clip which has a completely different sound.

Just posed this a possible cause of what he is hearing.

Q