I have in the past used a vpi brick (currently on top of my Ls7 pre) and it brought the hum down a lot. It may not do what you want on a electical basis but it is the cheap fix you may be looking for.
How to get rid of transformer hum?
I have a pair of Pass X600s and an ac line with about 5% clipping of the sine wave. This gives me a large amount of transformer rattle (mechanical hum). I have built a line filter with two diodes and a couple of caps that has gotten rid of 80% of the noise but I'm looking for a inexpensive fix. My power company will not fix the power coming in unless it is clipping by more than 7%. Any suggestions?
Tommy
Tommy