Resolved: The EP350 manual on the Axiom web site differs from my EP350 (I think mine's a v2). Anyhow, I was refering to the manual on the web site without realizing the manual that was in the mountain of paperwork I have in the house had a key element that the variant on the web site's manual doesn't have. My EP350 has a little screw on the back of the sub that, if removed, resolves ground loop issues. I removed it and the hum disappeared!