Ground Loop Hum

Hello, I am looking for help eliminating the hum I get in my subwoofer. I have a Rel B1 and am using the high level inputs. The Amp I am connecting it to is a BAT VK-600. Based on the documetation this should be treated as dual mono blocks so I have the two hot wire connected and the ground going to my pre-amp ground. I have tried different grounds including the one on the power conditioner. I have tried all the normal tricks of moving where the cable runs, unpluging all other componets, etc.

Any thoughts would be appreciated.