my one experience with a RB250 was it played fine without a ground wire, but if you have a ground wire attached to the sub-chassis of the table you can try to connect it to your preamp ground. If you hear hum, I would disconnect it and see how it does. If you still have hum you may need to fix a ground wire to the base of the arm somehow (the easiest way would be a wire with an alligator clip).
Separate question - forgive my asking if you already know this - the Rega arm has different geometry than the profile. Did you get a correct arm board for it?
Separate question - forgive my asking if you already know this - the Rega arm has different geometry than the profile. Did you get a correct arm board for it?