It's music being driven thru the amp at a level BELOW its clipping level, that is, within its power capability.
The only way I know of to determine what that is is to view the output waveform on an oscilloscope, not very practible.
The speakers are British?
.
The only way I know of to determine what that is is to view the output waveform on an oscilloscope, not very practible.
The speakers are British?
.