Regarding hum and the CJ preamp. I had a similar situation where I was using the AudioNet Max amps with a Simaudio 740p preamp. I use balanced ic's from the preamp to the amp. I upgraded my preamp to the Stern and suddenly there was a noticeable hum coming through my speakers. You can imagine how upset I would be at this situation. I could switch back to the simaudio and the hum would go away. I tried a lot of things in troubleshooting but eventually (within a couple days) I contacted AudioNet and they had me try a pair of single ended cables and the hum went away. Turns out that the Stern preamp lifts the ground pin in balanced ic's. I purchased a pair of ground cables from Shunyata terminated with RCA's on both ends. I simply ran this cable in parallel from the back of the preamp to the amp and it provided the gournd. No more hum.
You don't have to necessarily sell your beloved CJ preamp if you can find the problem that is causing the ground loop. But the process can be maddening.
Probably you were a victim of the famous "pin 1 problem" https://www.diyaudio.com/forums/chip-amps/269914-pin-1-a.html . Still an issue in many systems, it is one the reasons sometimes people experience big changes with grounding devices.