Radford STA25 Mk 3 modifiactions
a) I do not own, nor have I ever used any Radford Product.
b) This is a 6550 (KT-88) based amplifier, operating at the bleeding edge of what modern-production 6550 tubes can deliver.
c) From what I can see of Radford amps contemporaneous to that one, they are reasonably well made, if a bit crowded.
So, based on my experience with 6550-based power amplifiers (Dynaco, Scott):
1. The voltage selector is the least of your worries, assuming it is correct for your location. In point-of-fact, if it is a typical rotary switch vs. a fixed plug, it becomes a weak point in the system. Good that it is gone. NOTE: Dynaco and Scott typically changed voltage by reconfiguring power-transformer windings - permanent, IOW.
2. Using dropping resistors to reduce voltage to filter caps is altogether a bad idea from many perspectives. Putting them where they are seems to involve additional leads, exposing HV B+ where it should not be - another bad idea.
3. Nothing intrinsically wrong with 80s-vintage caps - I run a number of them myself. However, and since then, the chemistry has gotten remarkably better than before. If you are going to take on this device, consider a recapping with caps of the proper voltage and screened to be close in value channel-to-channel.
Cutting to the chase: you are offered an amplifier of reasonably good repute that has some hair on it. That, overall, you would like to restore to its original reliable condition and design.
Amp costs X
Restoration will cost Y in labor (presumably yours) and Z in parts.
X - (Y+Z) would be the fair price of this amp. Writing entirely for myself, I love a challenge and would think nothing of going through the necessary restoration steps. That is not everyone's position, however.
Good luck with it!
Melrose Park, PA