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

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.

I’ve never owned Quicksilver but have heard them many times.  I think the CJ amps better them in both build and sound quality.  If you like the EL34 power tube look for a Premier 11xs with Teflon caps.  They’re very hard to find so you’ll have to be patient. They’re also only 30w so be sure they will work with your speakers.  Premier 12xs is another more powerful option. I owned them. Also hard to find.  Both will be an excellent match with your preamp.  If you can find either amp with Teflon caps it will mean they’ve been updated, probably in the last 10yrs (or less), and will give you many more years of service.  The Classic 60 that you mention would also be an excellent option and would begin to give you the more modern CJ sound.

I've owned a CJ PV6 for 40 years. It has been in constant service. The CJ tech replaced/upgraded the caps to their new type in 2017. I played it 8-12 hours a day since it returned having been blessed by CJ. I just have had to replace the tubes. It sounds great and I would put it up against any modern unit that is comparable.

I have tube amps from the 50s, 60s, 70s, and 90s, plus a Had Inspire KT88 just for fun, and tube preamps from the 50s, 60s, 70s, and 90s.  They each have been restored to daily driver status, some with upgrades to the original intent, but I haven't heard anything in modern tube gear (within a budget of $4-5k) that compels me to replace them.  I'm sure there's something marginally better out there, but probably not at a price I'm willing to pay.  

I think its prudent to check electrolytics after 20-30 years. Its easy to visually inspect them for swelling and leaking of the liquid. If there is a leak, definitely replace. The first caps after the rectifier have the most stress, so might need new ones even if other caps are ok. I had a dynakit from the 1950s, and its cap was ok. I looked at voltages, and I looked at the waveform of the B+ at the cap tabs and could see the caps smoothing out the signal. I have a capacitor checker as well. So I don’t think age of a device makes replacement necessary as long as you like how it performs.