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.

Thanks
Russ