Let's do another experiment.
Disconnect your cable TV totally from the system by unplugging the video and audio cable from the wall or cable box and unplug the cable TV box if you have one as a ground loop can come from that too and can get in via the TV to the audio system connection or direct to the receiver from the cable in any and all ways. Once it is totally disconnected, see if there is still a hum. If not, there is a fix for that, if it continues, then it is through the power cable of the A500.

Then, try to run a ground wire from the chassis of the A500 to the other components, until the hum stops.
The let us know.