Sorry, was late and was tired. So didn't provide details.

I'm not entirely sure. First thing is I was using an EQ between the receiver and amp. That was increasing the noise floor by a few dB and also adding a ground loop hum. I reduced the ground loop hum to the level of the noise floor by grounding all the chassis together and to a ground hole on the power strip. This noise floor, with the EQ involved, was barely audible from my listening position.

But, disconnecting all inputs to the amp still had a noise floor that while inaudible from my listening position, was there.

Anyway, I removed the EQ, and this resulted in removal of something else besides the EQ: the RCA to 1/4" TRS Hosa cables I was using for interconnects. I am now using RCA brand RCA to RCA cables for the interconnects that I bought from Home Depot (including one labeled my green component video, since I needed 7 instead of 6).

This combination has resulted in an extremely low noise floor. Lower than when I shorted the inputs using 12awg copper or when the inputs had no connections. It is currently only audible from some of the drivers, and then only with my ear against the grille, and even then, barely audible. The chassis are still grounded to each other and the ground on my power strip.

Summary is the EQ was a noisy component, but also connecting the receiver pre-outs directly to the amp inputs using RCA brand cables instead of Hosas. The RCA brand cables seem to have better shielding. But that could just be them trying to make the cables look nicer.