Hi...the correct procedure would be on a test bench, hooked up to a load box, and being able to bring the amp up to power with a variac while monitoring the output with an oscilloscope. In most cases I monitor the AC current draw to look for impending doom.
Hypothetically, if a voltage regulator fails the distortion could go over 50 %causing both PG circuits to drop the input signal 40-50 dB.
printed circuit board