Electrolytic capacitors can hold a dangerous charge for months or even years after a device has been powered off, thus, it is always a good safety practice to discharge the capacitors before working on an audio amplifier

Connect one end of an alligator lead to the chassis ground, and the other lead to an 10K ohm resistor

Identify the power supply capacitors and touch the 10K ohm resistor's open lead to each of it's leads for a few seconds

If you are paranoid, you may check with a voltmeter to ensure that the capacitors are safely discharged

It should now be safe to work on your amplifier's circuit board