I would say you have a ground loop somewhere. I use 2 Stadiums, no hum at all. All on one curcuit. One time while changing speaker cables, there was a very noticable hum from one of the Stadiums. The ground wire was not connected yet and i forgot to unpower the subs while connecting the new cable. The amp and pre was off of course but the REL was on. After unpowering the sub and connecting the cables and powering up the system, back to normal, no hum. This told me a ground loop caused the hum. I would ck that all connections are secure and in order first. Maybe redue the connections from amp to REL and be sure no loose strands of wire are free. Ck your internal house line connected to the REL for a possible ground problem. Maybe connect the setup to a completly different curcuit to eliminate any possible ground problem in the curcuit. Disconnect one component at a time to isolate the culprit and see if the hum ceases. Hum is usally a ground loop or transformer interferring like mentioned above from others. Call REL and explain the problem, they should be able to help. I think you can call Sumiko in CA, Sumiko is the US distributor for REL. The hum you describe is not normal and should be eliminated. Good Luck!!