Perhaps the ground lead on your amp's power socket is not properly connected internally. It certainly doesn't sound like its doing its job as a ground. Try grounding the chassis separately, i.e., maybe run a wire from a chassis screw to the grounded screw on your electrical outlet. Good luck.
Help amp hum is driving me crazy
To start with I have a B&K ref20 pre with Polk SRT speakers. I have Kimber Monacle XL and 8tc for speaker wires and Phoenix Gold RCA cables. I use a Panamax 1000 and a DBS for protection. I had a Sunfire CG with no problems but felt I needed more explosive power for home theater so I bought a used Cinepro 3K6 II. It hummed like crazy at first (50db). So I began the sytematic aproach, I started unhooking items looking for the culprit. I eventually ended up with just an amp hooked to speakers, still humming. I tried an extion cord plugged into several other outlet in the house, some worse none better. The Panamax DBS works the best at filtering out the most. I tried cheater plugs in every combination I could think of to no evail. The ground lift switch on the Cinepro helps very minutely. I tried different interconnects. What else can I do besides get rid of it or spend thousands on filters I never needed before? I am an electrician and would like to think I am fairly well schooled but I feel like an idiot.