Thinking about it…
When planning the studio construction the first thing we did was making mains ground wire conns all in a star shaped manner. Quite usually those are often laid out in serial from one socket to the next. This introduces a difference in electrical potential causing hum. Some devices with asymmetrical layout don’t like this a bit.
Also, have mains ground of the house checked. If it is not zero Ohms towards the mains entry box to the house… problem…
We also build several mains circuits with fuses and residual current operated circuit breakers. Meaning, the control room with console, computer, rack devices, pre-amps, a.s.f., are on independent mains circuits and so is the recording room. The rest, like lights, gadgets outside the studio environment, aircon., vents, office stuff are on separate lines, too. In total we have 8 circuits with special star shaped grounding of all audio relevant rooms. On the backside of the racks we have a separate earthing cable, 16 Sqmm , connected to a Rackpotenzialausgleichschiene ??, where all cases and housings are connected to, as well.
Problems with noise and hum on stage or elsewhere is occasionally fixed with simply turning the mains plug. I am sooo grateful for those new light dimmers and LED lamps… 
Was a pain da bum in earlier years…
