sounds like you've got toroidal transformer hum - look for my thread titled "best tube pre for bryston 4BSST", where some folks put links to articles about this. I also made myself nuts doing away with it!
I still have some equipment with toroidals, and they are silent. But this one piece drove me CRAZY!
I still have some equipment with toroidals, and they are silent. But this one piece drove me CRAZY!