oldshool1,
I had the same problem to my Constellation Mono amp. Amp checked and find nothing at the Constellation repairing center. Checked with Shunyata Research, I used quite a lot electrical products from them, they said probably a DC on the main. With a reasonable low cost for a try, I tailor-made 2 DC blockers (one for each channel), to which to ensure sufficient current serving the Mono amp. No hum from amp anymore, and more importantly that would not affect the sonic performance.
I had the same problem to my Constellation Mono amp. Amp checked and find nothing at the Constellation repairing center. Checked with Shunyata Research, I used quite a lot electrical products from them, they said probably a DC on the main. With a reasonable low cost for a try, I tailor-made 2 DC blockers (one for each channel), to which to ensure sufficient current serving the Mono amp. No hum from amp anymore, and more importantly that would not affect the sonic performance.