I recently was dealing with a hum issue. A technician gave me the following troubleshooting checklist to follow, which worked very well. It is intended for monoblocks, but it can be revised for a stereo amp.
1) With all components DISCONNECTED from the AC mains electrical supply, make sure that the amplifiers are connected to the loudspeakers correctly.
2) Remove all 'cheater' / 'shorting' plugs.
3) Disconnect the preamplifier from the amplifiers, so that there is no input signal to the power amplifiers.
4) Use standard, supplied AC power cables.
5) Connect all components to the same AC circuit; source, preamplifier and BOTH power amplifiers.
6) Switch ON the Left amplifier ONLY, and listen for any hum / noise.
7) Switch OFF the Left amplifier.
8) Switch ON the Right amplifier ONLY, and listen for any hum / noise.
9) Switch OFF the Right amplifier.
10) Connect the pre amplifier (without any source connected to it) to the amplifiers.
11) Switch ON the pre amplifier.
12) Switch ON the Left amplifier ONLY, and listen for any hum / noise.
13) Switch OFF the Left amplifier.
14) Switch ON the Right amplifier ONLY, and listen for any hum / noise.
15) Switch OFF the Right amplifier.
16) Switch OFF the pre amplifier.
17) Connect a source to the preamplifier, preferable a CD player.
18) Switch ON the source.
19) Switch ON the preamplifier.
20) Switch ON the amplifier ONLY, and listen for any hum / noise.
21) Switch OFF the Left amplifier.
22) Switch ON the Right amplifier ONLY, and listen for any hum / noise.
22) Switch OFF the Right amplifier.