OK.
BTW - I'm not an electrician and I don't have nearly as complicated of a set up as you. Nor do I have any fancy amp switchers or Furman power conditioners. I toiled for months in trying to eliminate my extra noise and hum on my amps (Quick Rod + Mesa Rectoverb). That was mainly introduced when I hooked up both amp inputs to an MXR ABY box. I REALLY wanted this to work. Similar to what you are doing. I can actually run 3 amps through it but only 2 can ABY.
But this introduced a lot of ground loop noise. I tried preamp tubes, moving pedals around, replacing cables, etc. I WRONGFULLY assumed that the hum would go away if I put the amps on separate power outlets. Wrong. That made it worse.
So what I ended up doing which is working great is I have both amps (all 3 actually) on 1 power strip and one of the amps has a ground lift adapter on it like below. I can't believe a $3 plug adapter fixed all of my problems. What I learned is that when you have amps connected this way, 'electricity' is looking for a single path to ground. If you have multiple paths - you get hum and noise. So IIRC I decided to make the Splawn the ground because it has the beefiest transformers and built in safety (screen grid resistors) etc. Not sure if that is correct from an EE perspective but that was my thought process. Now that I look, it is my 3rd amp (Peavey Vypyr 60) that has the ground lift. Not sure if that means it is not grounded but I rarely use it anyway.
Not sure if any of this will help you but it doesn't hurt to try. I am back to 'normal' amp hiss and no extra hum or noise.
This is the video I found where I was like .... "Ah hah!" Go to the 3 minute mark at a minimum.
And these other ones might be good too - the one with the tree is grounding 101 which was decent IIRC:
https://www.youtube.com/results?search_query=fixing+ground+loops+in+guitar+amplificatin