Sub-woofer hum problem

Hi, I know this has been covered ad nauseum but my plebian brain can't come up with a solution. Here's the system: Wadia 850 cd player connected to an Eddie Vaughn Carina set el84 integrated amp in turn connected to Reference 3A speakers. I have a MJ Acoustics (REL) sub-woofer that is connected to the speaker terminals of the Vaughn via a Neutrik cable. I get really audible hum even when the amp is off and still connected to the sub. With the Neutrik connection disconnected from the sub there is zero noise, dead quiet, even when the amp and cd player are still plugged into the wall sockets. So my handful of brain cells tell me there is something not very synergistic going on between the sub and the amp. Any ideas greatly appreciated as I would like to use the sub to augment the speakers.

Thanks in advance, Cort Chambers