Yep, unavoidable, as far as I can tell. I've had two Regas with the same hum. My guess is I could reduce some of it by moving my preamp further from my amp, and some more by doing a rewire. But I doubt the hum would be eliminated completely.
Rega P3-24 w/ PSU, Groovetracer Reference subplatter & Delrin platter / AT-150MLX / Bottlehead amps: Seduction phono pre w/C4S / Quickie pre / S.E.X. w/C4S & MagneQuest iron / Klipsch Forte IIs w/ Crites upgrades / Sennheiser HD600