Tubed preamps and Power amps- How old is too old???

I had an all Conrad Johnson system including Premier 15, 16 and 12’s for many years of trouble free service. After 20 yrs. the caps may begin to dry out but chances are nothing to worry about quite yet. As long as you like the CJ sound I’d look to replace the Quicksilver amps with newer CJ amps. Quite a few you could choose from depending upon whether you like the older CJ sound or want to “modernize” a bit with the newer more neutral CJ sound. One of the LP models of their amps would match up nicely with the Pr17. You could also consider upgrading your preamp with CJ’s Teflon caps, unless you have the Series II which already has them. They’re expensive but take that preamp up several notches. Also try the Conrad Johnson Owners Group website to maybe get some other recommendations for specific CJ amps that fit your budget. I was just on the site and see where Lew Johnson just posted he will be reviewing what in his opinion were the 10 most significant products during his tenure. Maybe get some ideas from one of the founders.  Good luck.

Having the caps checked is a good idea. I have thought about CJ Power Amps. Being that I have tried a variety of output tubes in the Quicksilvers ( 6550s, KT88, and EL 34s ) I have come to prefer the sonic signature of the EL34 tube over the others mentioned. So something like a CJ Classic 60 would be a consideration. Can't say it would be an improvement over the Quicksilvers. Anyone compared them?

It does depend on the ones confidence around tasks usually undertaken by a EE.

I have experienced Valve Amplification where the foundation of the Amp' has been its very aged Output Transformers, that are repurposed using modern parts for the Schematic and Topology.

Any Amp' from a particular time frame can have a Circuit assessed for the measured accuracy of parts used.

Exchanging Parts for many is not too much of a chore or a concern.   

I recently tly purchased a 20 year old pair of Atma-Sphere MA-1s. They were purchased from a dealer and had been recapped and retubed. They sounded better than anything I'd had in my system or that I'd heard at a shop. My point is that some of that technology hasn't changed all that much. If you like your CJ then consider getting it recapped and after 10k hours maybe replace the tubes. 

well, the favorite conrand johnson preamps are PV5, premier 3 and PV8, which are...i dunno. late 80s and early 90s.

one of the consensus best is marantz 7, another is citation 1, another the audio research sp-3(A)-1.

serious tube people are still crazy about several oof the HH scott preamps, integrateds...maybe an eico also. i forget.

there are more to mention.

some of the best tube gear is vintage, some of it is a few years old, some of it is current production.

im not familiar with your conrad. 

ive had old tube preamps and found it worthwhile to service, maintain and increase the performance but improving parts quality and whatever upgrades are most appropriate. basic service is a recap, and it goes from there.