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?
I had a hum that came from a light dimmer switch installed in the wall. When I flipped the switch, the hum would go away for a while.

I also had a hum when using the VPI PLC to power my turntable. It broadcasts a lot of hum. You don't describe your phono setup, but if you have a PLC, it needs to be really far away from the cartridge and phono interconnects (and the preamps for that matter, as well).

There are probably others who may address possible ground loop issues in the electronics. Good luck.
I would've guessed a bad cap in the phono preamp's power supply that somehow got jiggled into function when you plopped the rig down at the dealer's....
If you're skilled I'd open her up and gently tap the caps and tubes with a chopstick (NOT A SCREWDRIVER OR ANYTHING METALLIC), and notice if you can elicit (or remove) the hum. I'm willing to bet it's not in your wiring, as your experimental groundig schemes should have eliminated it.
Good luck...and please don't yell at me, guys, for suggesting he go poking around in there. Chopsticks are ok.
You may want to look at the various connection points on your elctrical panel and water pipes for corrosion. Easy fix to brighten them up with emory sandpaper so they are nice and shiny so they make the best contact. Also make sure that there is a ground connection point located before the water meter (ie on the outside portion) of the water pipe. May want to look into whether the water pipe has at least 20 feet of earth contact (some systems actually adapt to plastic pipe). A final thought is to add a secondary ground rod at least 8 feet long and located at least 1 foot beyond your home's roof line. This additional ground needs at least a 6 gauge minimun copper wire bonded directly to the existing main ground.
Thanx for the suggestions, guys, definitely worth trying. I guess there's more that can be done with chopsticks than just eating sushi.

For the heck of it, I've got a Linn Sondek with/Valhalla & an Itok arm. Just replaced the cartridge with a Dynavector high-output MC - very fluid & musical when you ignore the hum.

To rid my tube system of hum (just so I could crank it to full gain to hear tube hiss with no stylus in the groove), I had to resort to grounding my tonearm separately to the preamp and then run a second wire from the 'table to the same spot. It worked the best for me to ground it right at the bearing in my old system. I was getting somewhere - the final step was to ground the preamp to my all metal stand! At high gain, there was a hum that would disappear when I touched the preamp and rack at the same time, thankfully no jolt or electrical buzz in the fingers. My system worked best grounded only at the preamp.

I noticed an interesting statement you made:
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.
This says that your turntable is not the problem (yet!), although the above advice is quite correct about ground loops in a phono stage and how *hard* they are to fix. You must be right about the hum in the house power. In my fog, I remember something about terminating the phono input with a suitable (50K? ohm) input load before testing for hum and noise. I don't know about your VTL 2.5 pre-amp. There are folks out there who build moving coil (MC) pre-amps for fun and they may have more details for you. Once the turntable is hooked up, the potential for new ground loops exists. Check out the post: impedance mismatched ?