All my humming issues were solved by one or the combination of 3 things, in no particular order;
* Ground loop isolator on the cable line.
* Removing the ground screw from the back of my EP500
* Making sure all the plugs for my A/V system were on the the same circuit or at least the hot wire was on the same leg in the breaker box (i.e. every other breaker in a normal US house, or every third breaker for those fortunate enough to have 3 phase power at home)
Just thought of a 4th hum, recently solved. When connecting video output from my laptop, I had to eliminate the ground leg on the power converter (3 to 2 adapter plug)
I have not reas through all your amp woes, but I would guess that you've already covered those bases...
