I had the same issue with a given combination of components, and never really resolved it. My preamp had mute, volume down, etc - all these things didn't stop the hum when the amp was on and the pre was off. Also, this didn't occur on all amps, just one. And that one amp didn't have the problem with other pre's. I really don't know. Have you tried floating the ground on various components? Unplug all the sources from your pre and see if the hum goes away. Then float the ground on your pre with a cheater and see the same thing. Please let us know what works, if you find it.
Best, Peter
Best, Peter