Cary SA500.1 mono blocks

Mechanical hum from the transformer is caused by DC on the line, which can result from many sources regardless of whether you have dedicated lines.

60Hz hum that you can hear through the speakers is sometimes a grounding issue and sometimes something else.

I believe the 500.1s accept balanced and single-ended inputs. Whichever you are trying, try the other. In particular, if you are trying to run balanced cables to the balanced inputs but your source/preamp is not fully balanced, and especially if you are using adaptors, then that could cause an issue. BTW, I also owned the 500.1s and they were dead quiet in my system too.

Is it mechanical hum of the transformers inside the amps, or a hum/buzz coming through the speakers? Does it come through both amps/speakers or just one? Do you have any similar (although maybe lessened) effect from your Rotel amps? Try a cheater plug and see if that makes a difference (indicating a ground issue). Hopefully, they are new amps and you don't have to keep them or pay for them. Good luck.