Hum issue - driving me more crazy

If you haven't run a ground from pre to amp, you need to try that and maybe with xover. If the gear doesn't have a ground terminal, loosen a cabinet screw and attach.

Try lifting the grounds on the amplifiers by using a two-prong adapter. You have the perfect setup for ground loops -- all equipment plugged in multiple outlets spaced far apart on the same circuit along with interconnects running the length of the room.

You really need to run the system balanced if possible.

Connect the A/C grounds along the same path the pre to amps interconnect goes. if it fixes the problem you can leave it that way permanently.
Even try the the neutral line same way. (but do not leave it, if it 'fixes' the problem, then you need to do some A/C rewiring) just leave the hot wire alone.
Sometimes electricians use the neutral wire in wrong ways and connect it up to other stuff instead of a pure separate wire. just the neutral A/C wire. (My refrigerator circuit neutral is connected to my separate 20 amp circuit neutral. Grrrr. But it does not seem to be a problem for me.)
Other possibilities: one of the items is wire wrong internally? hot and ground reversed?
Then also: run extension cords from ONE outlet at the preamp to both amps to test for buzz. If gone, then it is the A/C wires to that side are somehow currupted. IF no change with the extension cord, then it is not the A/C wire.

A comment about dimmers: some are great, and some are 'dimmer from hell'. I had one dimmer that would burn out bulbs in less than a two months. All the time. then another that the bulbs last for YEARS. same usage.
My understanding is some cut the current wave vertically and completely slicing it, bad lots of RF, some horizontally with a hard cut, OK, and some horizontally with a soft cut, (probably they just add a resistor, or a cap in the thing) and those are good, not so much hash.
None of them tell you what type they are. just from replacing bulbs you may know which are the bad type, as they kill bulbs faster. I took the good ones with me when I last moved!! (four years for the bulbs in the one room with the 'perfect' dimmer, and I use them every day!!)
Finally with your stuff you should get a pair of 14B-SST2 for those 4B-ST.(even 2 of the 4B-SST2 used would be great)
I have a 4B-SST2 with my 3.6s and it is really good. (I almost bought an ARC REF 3 used, but bought a Bryston BP-26 preamp.
Anyway, I like your stuff, and have similar stuff, so i 'more than usual' wish you well.
Good luck!

But there is constant low level hum - right side only EVEN with preamp and XOVER turned off.

I agree with Gs5556. Try some ground cheaters.

Just because you turned off the preamp and xover that would not break the ground loop current path through the right channel amp.

Have you tried to isolate the right channel amp from all other equipment? Nothing connected to the input of the amp... That would tell you if the low level hum is caused by the amp or not.