Hum from Silversmith interconnect

Swap L/R channels with the cables and see if it follows the cable.

I've also seen cases where the depth at which a plug mates to the jack makes a difference, but that's usually a sign of marginal jacks or poor mounting of the jacks/backplate design.

I've also chased this type of problem and found a bad joint in the terminations of interconnects before - visually looked perfect upon inspection, but upon mild flexing of the cable near the termination (while hooked up) it would "bzzzt" - it could have easily been the ground conductors touching the signal conductor only when flexed a certain way. Of course, "touch it where it hurts" debugging like that can be risky to your system... ideally, you have an old receiver+source, and some dusty old speakers somewhere so you can test them in a parallel system.

If the terminations unscrew, visually inspect the terminations/solder work.

You probably have RFI/EMI or ground loop issue in your setup. Since the cable is not shielded, it is very like to act as "antenna" which applifies the interference
Either fix the problem or dump the cable