You don't mention is the hum isolated to one channel or both? If both double check your cartridge connections make sure they are not crossed. Also check the path the cables run in. I have a RB600 with Incognito and i noticed that the cables don't have the sheilding the OEM Regas did. It may be that you need to re-position the cables. If it just one channel and it disappears with a ground to a RCA plug then try grounding directly to an outlet screw and see if that does it. Also I think what Dan ed was asking do you have an ohm meter and if so check all your cables from cartridge to RCA plug and make sure you don't either have a pinched wire or bare wire. All readings should be within 10%.
Incognito rewire problem
I have installed the Incognito tonearm rewire on a RB 250 Rega arm with the Origin structural mod.There seems to be an earthing problem as I have a continuous and loudish background hum not present at all previously.Checked the wires in the tonearm and base,they are not broken.Could it be the cartridge? A Sumiko Pearl that now hums when near the Planar 2 motor.Thanks,any help appreciated.stefanl