60 cycle transformer hum in only one of 2 tube preamp/compressors. Remedy?

I have 2 very hi end tube preamp/compressors; and I am experiencing an issue with some audible 60 cycle transformer hum (not in the signal chain, but hum from the unit itself). Both preamp/comps sound and function identically on signals, but only one of them is exhibiting this hum, so I assume there's an issue there.

I have them both in a dedicated rack away from other gear. I've tried reversing the layout of the 2 preamps (one on top, vs the other), checked all cabling, etc. The hum still follows only one unit and it's always the same unit.

Any idea of what might be causing this, and is there an easy fix? It's a bit annoying, as I keep the preamps close to my listening position, so I have to turn it off when doing detailed listening. Not causing any issues in my recordings, and I keep them far enough away from the mics, but it would be nice if they were both equally quiet.

Thoughts and suggestions?