The folks at BAT have a great service dept. Victor Khomenko himself will most likely answer any of your questions you may have concerning your VK-30.
Have you tried floating the ground? is it coming from both channels? If its only one channel it is either a tube or capicitor, if its both channels I would put money on it being something VERY simple to fix-i.e. new/different cables or floating the ground. From my experience I have found a great deal of change in the sound with various PC's on BAT gear, perhaps if you have any other laying around you could try them as well.
You don't have any ties to a cable line, do you? Meaning, you don't have a vcr/tv/Tivo or any component that may touch an incoming cable line connected to the BAT, do you? If so, unplug the inputs into the back of the BAT. Tube preamps are notoriuos for picking up hum from a cable line. I had a problem with a Cary and an Audio Research before I realized I could not hook my VCR up to my preamp.