I'll try to explain, which means I just may botch this up. :-)
The hum was only between the two amps. Everything upstream unplugged. Only the amps were on. Ground loop between the two amps. The cheater plugs proved this.
Now, amp A had a ground connection from both input (-) rings over to chassis ground as well as the (-) wire going back to the power supply. Amp B had no such chassis ground on the inputs. Both amps have chassis ground from the transformer.
The transformers in the power supplies of the two amps don't tap to identical potential. So you get a ground current between the transformer taps on both amps and it hums like crazy because there are two paths to ground through both amps due to the leakage. Remove the connector ground to chassis (which is really the transformer tap) and no more leakage.
The hum was only between the two amps. Everything upstream unplugged. Only the amps were on. Ground loop between the two amps. The cheater plugs proved this.
Now, amp A had a ground connection from both input (-) rings over to chassis ground as well as the (-) wire going back to the power supply. Amp B had no such chassis ground on the inputs. Both amps have chassis ground from the transformer.
The transformers in the power supplies of the two amps don't tap to identical potential. So you get a ground current between the transformer taps on both amps and it hums like crazy because there are two paths to ground through both amps due to the leakage. Remove the connector ground to chassis (which is really the transformer tap) and no more leakage.