Subwoofer has a Hum

I have two Martin logan Descent subs. The one sub has a slight hum, the other one is fine. both are hooked up to my Bel Canto pre2p preamp through rca outputs.
I tried changing the pc, used a cheater plug to by pass the ground. I have all dedacated circuits for my gear. There is no hum in my main speakers which are the ML Ascenti. The sub hum is reduced when I remove the ic going to it. I tried to switch the ic's, use differant outputs on the preamp. I still have the slight hum coming from only the one sub.
Can anybody give me some advice in this matter.

Cheater plug is the easiest fix...If you don't like the looks of those things try a Monster Power HTPS 7000. I am amazed at how well this unit cleans up the hum.
Get a ground loop isolator, I had the same problem with my sub and my tubes. I got mine from parts express.
"I have all dedacated circuits for my gear."

And therein lies your problem. Plug everything into one circuit and be amazed when the hum suddenly disappears. Try it and let us know. I'd be very, very surprised if that didn't solve your problem.