“The Bluejeans Cat 6a cable has a floating shield design which may help with breaking a ground loop through the shield.”
Tripplite ISO 500. https://www.tripplite.com/isolator-series-120v-500w-isolation-transformer-based-power-conditioner-4-outlets~IS500
If you removed all equipment to the smallest state, and you still get hum, then something is broken. Do the high-tech thing and jiggle the two wires left in the system (on both ends). Then power off, pull the covers, and under a bright light, look for a solder joint break at the RCA jacks. Flex the leads etc with a plastic spudger to open/close the crack.
Before you do that, try running a long long extension cord to another faceplate that you know is not on the same circuit. Also, (so obvious I didn’t ask), move your equipment 3 feet in any direction… just to be sure you are not some emi field.
Peace
Bruce in Philly