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

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.

I agree. You really need to be asking this question on the Conrad Johnson Owners group. There, you will find a lot of very knowledgeable audiophiles who have experience with CJ equipment. The LS17 is a classic. At one time, it was highly regarded by many users. If you are interested in having it serviced, call CJ. For what it’s worth, I would only have CJ service the unit. I’m a longtime CJ owner - amp, preamp, and CD player. 

There is no simple answer to this question.  if it sounds good, i would not do anything to it until it shows signs of needing work.  I know of many amps and linestages that are decades older than yours that work perfectly well.  A friend has a terrific sounding amp that is about 75 years old.  

I have a pair of Luxman MB3045s from 1974 that still sound spectacular.  As mentioned, tubes and caps are really the only parts of concern.  Have it tuned up and brought back in spec and it may out last you.

IMO after 15 years capacitors especially start drying out ,they may still work 

but you can loose well over 15-20% of its true fidelity

having been into modding for over 20 years i know 1st hand, and rebuild my own 

Loudspeaker Xovers since the vast majority from the factory use at best average quality ,they are never-concerned about the Absolute best unless you are spending 3-4 x the average per component .8 owned a Audio store for a decade so speak through experience.