lower voltage requires higher amperage ( to make the same output ) which creates higher heat and exceeds the internal components thermal limits...
some direct quotes from people with degrees in the field...
Current=Power/Voltage
As voltage goes down, current has to go up to maintain the power. You will get to the point where the input section of the power supply can't keep up and **POOF** out comes the magic smoke
Power = Voltage * Current
Chris is correct, To maintain the voltage rails on the output of the power supply section, the input power must stay constant with the output. If the input voltage goes up, less current is needed, and if the voltage drops, the current goes up
If an amp is designed such that it does not shut off when too much current is being drawn by the power supply, that is just poor design. There are capacitors on the output rails of the power supply that supply current peaks to the amplifier section for high transients in music. If there is not enough capacitance and you keep running the amp beyond it's rated power and it doesn't shut down, somethings gotta give
and there is always the bible...
Basic Car Audio Electronics