I had this exact same problem for awhile. I lived with it for about a year but was recently prompted to investigate further.
I found a solution here-
http://www.audiokarma.org/forums/showthread.php?t=86591
"The preamp is fixed. I found that there was a ground loop between the case and the two screws that hold the circuit board to the case. So I removed the ground connection to the PC board near the power indicator LED and the hum stopped. Also I put a star washer between the top and bottom case to insure there is a good ground as the top was not grounding well to the bottom. IMHO the preamp sounds very clear and open."
I simply opened the unit and placed a small piece of electrical tape between the PCB and the place where the screw fixes it to the case nearest the LED. I then left that screw out, the other screw and the screw at the rear hold it all together well enough.
The hum is gone and I noticed an immense improvement in sound.
I found a solution here-
http://www.audiokarma.org/forums/showthread.php?t=86591
"The preamp is fixed. I found that there was a ground loop between the case and the two screws that hold the circuit board to the case. So I removed the ground connection to the PC board near the power indicator LED and the hum stopped. Also I put a star washer between the top and bottom case to insure there is a good ground as the top was not grounding well to the bottom. IMHO the preamp sounds very clear and open."
I simply opened the unit and placed a small piece of electrical tape between the PCB and the place where the screw fixes it to the case nearest the LED. I then left that screw out, the other screw and the screw at the rear hold it all together well enough.
The hum is gone and I noticed an immense improvement in sound.