Audio Alchemy OM 150 hum. What is the cause?

I recently purchased an AA OM 150 at a really attractive price. The former owner disclosed that the piece had a hum coming equally from both channels. I hooked up the amp and sure enough it has a pretty hefty hum from both channels. When I disconnect the interconnects the hum in both channels disappears. Connect either one alone and there is no hum. What gives?