[Diabolical Artificer]

It depends what R2R your talking about. If an old Ferrograph or Brennel, then the valve amps in these are a large part of how it sounds, and thus the reason why a lot of folk like them. If a Revox or similar pro R2R, I don't see that you can do much better than the designed amp, they knew what they were doing. Changing a few trannies for better quality low noise parts might result in lower S/N.

However, to a point, a different amp for consumer or similar R2R like an Akai or whatever might be worth looking at, but really as others have pointed out you have to look at the hole signal path including heads and probably a great contributor of signal degradation, the tape transport.It really depends on what your trying to achieve.

Andy.