check the ventilator behind ...
if the vent is blocked by dust
or the motor is no longer turning properly due to lubrication issues
or the motor has lost contact ( there is a set of contacts inside from the rear panel to the stand by transformer pcb)
or the vent is simply gone
In any case of the above reasons protection circuits will notice that and will not allow high levels of music .
That is a precaution in this amplifier far before high temperature even begins to built up .