Early docking sleds [earlier than about serial number 1050] had no earth wire on the serial cable that connected to the docking sled.

One fix [and probably the easiest] is to make up a serial male/female DB9 connector pair that plugs onto the end of the docking sled serial connector and puts all the 9 pins straight through and also connects pin 5 to the metal chassis of the docking sled.

That may make your hum go away as it sounds like you have a earth loop between your PC? and your Empeg.

Of course, your sled may not have the missing ground wire problem, in which case, it could be a earth loop caused by having the grounds connected between your docking sled and the PC - in which case try plugging the PC into the same electrical outlet that the Empeg power supply is plugged into [this should help eliminate the ground loop problem I think].