I know you've found a solution already, but I would also have kept the offending transformers in place and disconnected all the audio connections from the mixer. Then replace the audio connections one by one until you find the one susceptible to the hum. Or is it the mixer itself that is susceptible?
It's been a couple of days now so the information has already left my brain... but basically, I don't think it mattered what was plugged into the mixer, if those transformers were plugged in under the rack gear, there was hum. Also, there was another load of noise getting in from the dodgy outputs of a tape deck that I'd forgotten was patched in to the back of the patchbay and thereby to the mixer - with the tape deck gone and the transformers moved, it's fine.
I did indeed remove all the audio connections one by one and re-try them individually and accumulatively before moving the transformers, as I'd originally though it to be a ground loop rather than induced hum - I can't recall now whether particular channels were more hummy than others, but I had not realised at that time that the tape deck was chucking crud out as well - that was when I decided to remove everything and start again, one piece at a time.
As for the mixer being susceptible I don't know. It's an A&H Zed R16 so I'd expect it to be decent. It sits just to the right of a 4-foot-ish rack (a couple of Quicklok units atop each other, so all metal with perforated sides and crossbars at the back). I've about 20U of gear in there, most has a regular 3-pin UK plug, three units (as above) have external supplies, and only the Boss uses a step-down. I only switch on what I use at the time. I've strapped the mains cables together down one side of the rack, and they branch off to each unit at height. The audio to/from the mixer is a 16-way unbalanced loom that comes into the back of the rack at the top and then descends to the patchbay on the opposite side of the unit to the power cables. Remaining audio and MIDI connections are somewhat free behind the modules, with a couple being tied together for convenience.
I don't honestly know how the hum was getting into the audio chain, as the transformer units were on the floor at the base of the rack and not near any audio cables. They were in the middle of a fair bunch of mains cabling though.
None of the rack gear is isolated from the rack itself - it's all just bolted in.
The whole stack goes back to one wall socket.
In short, I don't know enough to say exactly why these various noises were appearing, but they were, and now they're not. Hopefully I can keep it that way!
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