If they are plugged into separate outlets try plugging them both into the same outlet. You may be getting a ground potential hum even if both outlets are on the same circuit.
What gives with my Monarchy SM 70 pros?
I just recently purchased my second sm 70 so I could run them mono to drive my Usher be 718's. I am getting a loud hum in both channels. Has anybody had a similar problem and solved it? I might add that I am running them unbalanced.