I am having the same problem with my McIntosh C-2200.
I had a dedicated AC outlet installed. I still have the hum. I used cheater plugs and it reduced the level of hum. I notice that with my Velodyne DD-15 hooked to the aux pre-outs, the hum is present with the pre-outs switched on, but not present with them off. I have used different patch cables on the DD-15, but still have the hum.
The hum only happens on tube warm-up. When the C-2200 is "warmed up", the hum goes away...
I'm going to Radio Shack 1st thing today to get the ground isolation transformer. In any event, it can't hurt, right (?)...