Thanks, but that didn't fix it either. I noticed that no matter what I did with the master volume knob on the HD, the hum did not change at all. It turned out to be what I believe to be a ground loop issue. I had the HD and the AMP both plugged into the same wall outlet. Different sockets, but the same outlet. I moved the HD to a socket on the other side of the room (I had to use a longer guitar cable because of the rediculously short power cable) and the hum was gone.

Any tricks for eliminating ground loop hum when plugged into the same wall outlet?