Used vs new Tube mono amp? - same model

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

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.  

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

I agree with the sentiments expressed above. If the manufacturer has not made any notable changes I don’t believe that you’d hear any difference between them. I do also agree that it’s dependent upon one’s personality. Some people are more obsessive compulsive than others. Do know thy self.

Charles 

I've two systems with two sets of vintage tube mono blocs from the 50s. They are not matched pairs but they sound the same and I've never even thought there might be an issue. 

If you believe in equipment break-in, the new amp would need several hundred hours.  Once there, if a new set of matched tubes was put into both amps, they should sound pretty much the same.

Sometimes manufacturers make "running" changes to the model without really saying anything.  Sometimes it's minor, but you never know.  I would check with the manufacture to see if there were any.  Serial numbers would be most beneficial.

How much are you saving? 
 

If it i could afford it. I would get the new pair. Tube cycles are in synch. Sure, would not sound different. I buy audio equipment as expensive as I can afford and keep it for at least ten years. I would prefer they were both really identical.

This has a lot to do with your personality.

Assuming the older amp is performing to spec it really shouldn't make any difference.