It's a ground loop, so solve it the way you'd solve any other ground loop (with lots of trial and error).

Something to know about the empeg's serial port: You only need to use pins 2, 3, and 5. That's all I've got connected in my home docking stations, and it works well without inducing any ground loops. Maybe that's all you need is a connector that only touches the empeg with those three pins.

Another thing to try is to use a different power supply.