I just saw your post noting that moving back to the first amp stops the hum.

Are all your amps located together, and connected to the same phase of the power line? If not, that could be a source of hum by introducing a voltage difference onto the shield of the audio leads between chassis grounds.

Do any of your components have three-wire power cords with signal ground connected to power ground? This could be its own problem.

Judging by the lack of hum when you get back to the original amp, you will lose the hum if you use transformer isolation between the multiroom output and the amp inputs. This is admitting technical defeat, and I will always pursue this problem to the end (at least I did when I worked for a corporation where the boss wanted to know WHY, no matter how long it took; these days, I just install the transformers so I can collect on the job).

There's also about 2.78 chances in hell that interconnecting the grounds of the preamp and the offending amp will lower the hum.

I will look up a GREAT hum-troubleshooting process at Jensen Transformer and provide the URL after I send this note.