With the wooden decks, like the Regas, there apparently isnt any reason to ground anything but the tone arm tube and the base. The motor and main platter bearing are isolated because everything surrounding them is wood. So the ground wire to the phonostage only connects to the arm tube and the arm base. Well, actually Rega doesnt even use an external ground wire, they ground the arm to one of the RCA shields (single shielded coax). Since the Technics SL1200 has a metal deck, I think they added the ground to the chassis (grounded at one of the pitch slider to base mounting screws) along with the arm tube ground and the arm base ground. Anyway, it works, they are generally nice and quiet. But the tonearm wiring is pretty poor and it connects to the external cabling through a PCB in the arm base. Stock internal tonearm wire is a stranded, tinned copper as well. So, it's generally accepted that a tonearm re-wire with better quality wire is a worthwhile mod.
Seeing how it is quiet as it is, I would expect that it should remain quiet retaining the grounding scheme. But I asked about the Quad in this case just to make sure that it should be better at noise rejection than the single shield coax. Or atleast, not worse seeing how it is quiet in the stock configuration. About Pauls idea for connecting the external ground wire to the quads shield instead of a run all the way from the arm base ... I didnt understand his explanation either. But it seems counterintuitive from the perspective that the shield would actually be physically connecting at the pre-amp end doing it that way. It's electrically the same from a strict sense, but as you mention, noise issues can be tricky and illogical. So I may stick with what I know has worked in the past.
Thanks