The filter caps may be drained already but you must insure that any deadly voltage is removed from the amp before you begin.

Clip one lead from your multi-meter to ground on the chassis ground and with your meter set to Volts DC, measure from the point indicated by the arrow in the picture. If the voltage is above 10 volts you must drain the filter caps.

Using an alligator clip, attach one side of the 330 ohm power resistor to the chassis ground and using insulated pliers to hold the 330 ohm power resistor, touch one lead of the resistor to the point indicated in the photo above. Leave the leads in contact for 30 seconds and measure again. Keep repeating this step until the voltage reads below 1

Capacitors can recharge even when the amp is not plugged in. After the filter caps are drained, take an alligator lead and clip one end to chassis ground and remove the red lead just above the place where you measured the voltage. Clip the other side of that alligator lead to that spade. This will prevent the filter caps from recharging.