Grounding sounds like the culprit here. Here are a few things you should check:
- See if the PC and the stereo/mixer setup on on the same grounded circuit (Same power bar or same wall circuit)
- Check to see of the mixer is a grounded device? Does it have a three prong plug (North America, not sure of UK/Euro plugs) If not, perhaps ground it's chassis to the amplifier, simple 20 gauge single strand should suffice.
- Grab a buddies voltmeter and check to make sure your PC chassis is a proper ground, and make sure the systemboard in the PC is properly grounded to the chassis as well.

If all that doesnt solve the problem, then perhaps running the simple 20 gauge wire from the chassis of the Empeg unit to the chassis of the mixer/amplifier setup should do the trick.

Have fun! Or just simply disconnect the serial & USB cables when playing at home.