Ground Loop?

Luxman 507uX.

I don’t see any indication the amp is Class II Listed. Can some one point it out...

Funny thing is, I didn’t remember seeing that either. I own exactly that model. :) I’m not breaking my back to find out. :-) AFAIK, double insulated gear has a dual square logo and I haven’t seen that. I’d have questioned if it was even legitimate based on how the unit is constructed.

Forgot to post the blog with the images I referenced, my bad:

https://inatinear.blogspot.com/2023/04/luxman-507ux-empower-project.html

There is a separate blog about how it seems the Luxman has the wrong leg fused here:

https://inatinear.blogspot.com/2023/04/luxman-507ux-ac-inlet-mistake.html

@invalid I'm not an expert in all things that need to be tested and approved, but based on my amateur reading of the requirement for a class 2 insignia, the device needs to be double insulated, and, again, from my eyeballs, it's no better insulated than any other power amplifier.

Please see the middle picture where I show the bare fuse holders, with the AC cables to the right of them.  That is what you would see immediately upon removing the bottom panel.

To my poor unqualified eyes that is not a class 2 cabinet.  Maybe some one else can help me understand how it is.

@invalid AFAIK, the equipment ground is not an NEC thing but a UL thing.

AFAIK, you do not have to ground a metal chassis IF the power supply is double shielded from the chassis. The Luxman is not double shielded.

What does internally grounded even mean?

I was pretty sure the OP meant 20A wiring, so I just skipped right over that. :) 

One other thing to check is that you have a good neutral.  Use a multimeter to test from Neutral to Ground.  Should see no more than a couple of volts.

Do you have any other metal to metal contact going on?  Like via your shelf?

Work your way back from your amplifier and disconnect all other inputs to your main preamp.

If you just have the JC3 Jr, with no inputs, and your preamp and amp do you get a hum?

Also, if given the chance to use the balanced connections, do so and lift the ground on the destination if possible.