Ground loop hum

I recently purchased a VTL 2.5 pre-amp (with integral VTL phono stage) and a VTL-ST85 power amp. They sound gorgeous, except for the grounding hum which appears only when phono is selected on the pre-amp.

This hum is volume-dependent and not insignificant, and is present even with nothing connected to the phono input on the back of the pre-amp. It is not present when any other source is selected. I have tried lifting the phono ground wire, cheating the ground on all power cables (turntable, pre & power), powering each of the individual components from different circuits, and most recently, running a dedicated audio circuit using #8 stranded copper. None of these has helped.

Interestingly, the hum does not exist when my entire system (including interconnects and the AudioPrism power conditioner I use) is hooked up at the local audio shop. So the problem is definitely in my house. My panel is grounded via a water pipe. Any ideas or suggestions?