Speaker hum.

Been out of the hi-end scene for a while, but finally did what I've wanted to do for some time-get back some into equipment I regret selling years ago. Anyway, recently picked up Krell models KRC-3 preamp and FPB-200 amp. Yes, they are old, but after gyrating for years with other models and brands, I found that I preferred the design of these. Anyway, set them up shortly ago and have a very subtle hum coming from the speakers (an even more elderly pair of Mirage M-1s). At first, I first had a conspicuous hum coming from the preamp, but that turned out to be caused by some incandescent lamps/lights using the same outlet. I solved that with some LED bulbs, but I then noted the hum from the drivers. I expect the hum is form an AC power ground loop, but beyond the equipment sharing the same outlet, I am wondering how to best solve this. It is hardly noticeable, but you know how that is. Thanks for the help.