Dear Mike, Do you really think that the differences you hear are mostly due to the tonearm/turntable interaction (consistent with your statement that mounting the Reed on your Garrard might give you the best of both worlds). I am kind of biased toward the view that the tonearm/cartridge interaction is dominant, so it may be that the Reed is just a better ride for the A90. Probably the lower noise floor you perceive with the Mk3 vs the Garrard IS due to the inherently more quiet operation of the Mk3, however.
At a lower level, I am trying to figure out words to describe the differences between my own SP10 MK2A in a natural slate plinth of my design and my Lenco remounted on a PTP in its own natural slate plinth. The Lenco is really dead quiet, so I cannot say I hear much difference in terms of noise. I need to swap the tonearms back and forth to get a better bead on what is happening. In my case, I do not have the same cartridge mounted on both tables, either. What is remarkable is how much the two tables sound alike (in a very good way), once mounted in slate. I have heard people say that the Mk2 is "sterile" sounding, especially lovers of idler-drive turntables, like Jean Nantais. I know what Jean meant by that remark, but in slate that quality disappears and the sound opens up and gets bigger and more lush. I can't wait to hear the Mk3.