The power source on the headphones is an AAA battery, so not much can be done there. I tried switching from a rechargeable to a fresh alkaline AAA, as well as changing the outlet the AC adapter was plugged into, but noticed no change. I will bring in my spare AC adapter tomorrow and try that. One thing that I discovered that does help is to keep the serial cable attached between my laptop and the empeg. This significantly reduces the hum, but does not eliminate it completely.

The ground loop RCA filter looks promising. Hopefully I will not have to resort to connecting its ground cables to me with electrodes in order to isolate it properly. My thanks go to Matthew, Paul & Tony for your insightful suggestions. I'll let you know what ends up working. ...Mark