Here is the thing. Tech's figured out a long time ago that sprinkling a little holy water on the amp in the form of "adjusting bias" was fun, easy, and lucrative. That's not from me, that's from Randall Smith, founder of Mesa Boogie, and he doesn't put adjustable bias in his amps. He does sell tested matched tubes, though.
Anyway, as ClintJ points out, the bias is absolutely testable - a tech can somewhat easily test the amp and determine if the bias is acceptable. There is a somewhat wide range of current that is just fine in the tube, and as I suggest the tube is not being run hard so it will probably be right in the window of acceptable. If it is, you've tested it and you know it's fine. Mostly tubes get into trouble if there is too much current. If you change the tubes and the tremolo works, I would call it good. If you change the tubes and it sounds the same, I would call it good. I would probably choose brand new matched tubes from a brand name supplier over random vintage tubes for everything, although I do happen to be working my way through a batch of vintage RCA's my wife found in a rummage store for $2 for a big bag of tubes in original packaging. If you get it tested with Groovetubes, for example, just keep buying Groovetubes for peace of mind.
Anyway, that's all I got on this
