Hi Doc,
I should probably clarify my PR99 / Studer remark. If you compare the transport mechanisms of the Studer B67 and A810 to the transport of the Revox A700, B77 and PR99 you'll see that they all share the same concept and manufacturing for the chassis, the motors, the head block and the pinchroller mechanism. The main difference between Revox and Studer is that the Revox machines are simplified to a bare minimum to reduce cost. Most other prosumer decks are cost reduced by compromising on construction and component quality.
The B77 and PR99 don't have regulated tape tension, which compromises the wow and flutter at the end of the tape and they don't have a scrape flutter roller, which I think was not a good cost trade-off, but they still have the same rigidity and stability of the mechanical construction and the same head performance as the Studers, which makes those little machines remarkably good performers, especially with updated audio electronics.
The absence of a scrape flutter roller is not very noticeable with Agfa 468 or Ampex 456, but with Quantegy GP9 and other 3M tapes it is certainly audible and measurable. I think they should have invested the extra $2 or so for a scrape flutter roller.
So to come back to the original remark. The PR99 is basically identical to the Revox B77, both mechanically and also with respect to the REC/Playback core. The PR99 was actually developed as a professional version of the B77, because so many B77s were used professionally in the broadcast industry.
In Europe Studer/Revox was known for listening to the feedback of their customer base, hence the mkII and mkII evolutions of many of their machines. The PR99 was completely a result of that customer feedback on the B77. It was actually not at all an obvious product for the Revox brand back in 1980.