Ok, Njonker and Jmc have good advice. Try lifting the ground (use a cheater plug) on the amp and then the preamp, but my guess is that wont do it, because I dont think you have a grounding problem. I have tube preamps that in combination with other equipment put out the same kind of hum. Now my stuff is not as expensive as yours and it surprises me that a Bat combo would, but it may be something else in or adjacent to the system. I have a vintage Luxman tuner that just plugged in, not even connected or turned on, will produce a hum through my system. (Anyone want to buy it?) In my second system I have an inexpensive Anthem preamp that in combination with my old Muse 100 amp puts put the same low level hum, and some people say it does the same with the Anthem amp, but its perfectly silent with any other amp I hook it up to. And the little Muse is quiet with any non-tube preamp. Not a good combination. You might also try plugging the preamp into a different circuit altogether if you can. Use an extension cord just to see if that eliminates the hum.
Then, how long have you owned it? If you bought new, have you talked to your dealer?
