Rega use the signal gnd wire on one channel as the gnd. Which is fine in single ended phonostages which throwaway signal - on both channels by grounding them to the phonostage chassis.
But step up transformers don't do that, they use the full voltage swing between signal minus and plus on each channel. They retain the fully balanced signal that comes from the cartridge. Rega assumption that they can do what the hell they like with one signal - from the cart is the root of the issue.
You need to rewire your tonearm. No amount of fiddling will otherwise fix this. As long as signal- in one channel is used as earth it'll always hum.
Kuzma Stabi/S 12", (LP12-bastard) DC motor and optical tacho psu, Benz LP, Paradise (phonostage). MB-Pro, Brooklyn dac and psu, Bruno Putzeys balanced pre, mod86p dual mono amps, Yamaha NS1000m