Used vs new Tube mono amp? - same model

I assume you will have a preamp with the ability to adjust for any minor differences in gain, you'd need that even with a new pair.

Jerry

Depends.   The Quicksilver Mid Monos have gone through three running changes.  First version has large OPT , one bias VR pot , single LED.   Second had smaller OPT,  one Bias pot ,single LED,  third version has a separate VR bias pot for each tube....  and test jacks.    I don't think I'd mix and match those.  

It should be fine if the circuits are the same, and you use identical tubes of identical age in each.