Adcom GFA 555 hum. Probably a repair question.

I picked up an Adcom GFA 555. I have placed the amp in two different systems, at two different residences. The amp hums.
It doesnt make any noise just sitting idle with the speakers connected and no inputs connected....but as soon as RCA cables are plugged in (connected to either of two pres or just the cable to nothing) the amp exhibits a 60 cycle hum. This has been taken out of my house and retested to confirm at a friends. Same thing.
Already tried lifting the ground to the whole system. Already verified no dimmers. Unhooked cable entering house. Used a speaker wire to ground the amp chassis to the pre chassis. Nothing fixes it.
I need some more suggestions. I am handy with a multimeter and soldering iron if there is something inside to test and replace.
Make sure the grounds on the RCA's are not broken loose. A broken ground anywhere in the unit will cause that hum, but only when RCA's connected tells me it is a RCA problem. Either in the cables or in the amp. Just use your meter between the RCA's on the amp and see if it reads 0 or close.