@ericsch Due to the fact that your other phono preamp also had a hum (at 50% volume instead of 25%), the JC3+’s previous owner did not have any issues, as well as many other pieces of evidence from the tests you just performed... My conclusion is that the Cayin is at fault.
Cayin is either causing a ground loop due to improper grounding scheme, i.e. RCA jacks not properly isolated, signal/power/Earth all mixed together, etc. Or one or more of the components is failing, such as tubes, electrolytic capacitors, etc. Please note, most, if not every single part in the Cayin is a counterfeit.
The next test would be to try another integrated, or receiver.