Man, I hope the metal shelves with all those preamps are grounded.
This reminds me of old - 1970:s - method that I did read either in Studio Sound or dB-Magzine way back then:
If you have hum in your studio, and normal procedures can't fix that - there's still one more method that's going to work:
At the doorway of your studio - and all systems powered up - make your choise of left or right - then start creeping with side cutters - each and every cable that you come across - cut the groundwire - if the hum gets louder, solder it back - if the hum gets quieter, leave it open. This way you either get a quiet studio or get electrocuted.