Lifespan of amps and preamps?

There are two questions here, I think:
1. are they sonically obsolete? 
--> in my opinion, no.  There's lots of great 30 year old stuff, and most importantly you said you like it. QED.--> there may be better stuff, but i'd go very cautiously
2. Can it be repaired? 
--> with solid state gear, 95% of the parts will last 50-100 years--> however, a few very important capacitors are likely near their limit. If and when they fail, it could be catastrophic.--> yet, even with a major melt down, it probably wont kill the rest of the amp. The biggest casualty might be speakers which could see DC or transients.  Are your speakers fused?  I really do advise fuses.....about 1 1/2A fast blow.  Amazing how LITTLE they degraded the sound.  After ripping them out, holder, wiring and all, i put them back in (as in line adapters on my speaker wires)
-> re-capping is not rocket science, but it is laborious. Maybe not worth it.--> while not catastrophic, old power supply caps in particular will slowly degrade the sound. Amazing how good some 30 year old products sounded when i oversaw the re-capping of a dozen or so of them with new, better parts.
Adcom is probably pretty easy to re-cap, relatively.

I agree that you really should look at the modern world of HD streaming, and a good DAC. Need not cost all that much for the DAC there is one i think is a steal at $99 and made right here in the USA.  Got a laptop? With a DAC you have a streamer then.  And computer streamed FLAC/ALAC files sound consistently better than the same thigns fro a CD - not as one might guess, worse.  There are  good tech reasons for this. (of course it must be done right, beyond this discussion).
One more story. I had to clean out my hosue to do a rebuild. I pulled out an original NAD 3020, spica TC-50is, and an AR turntable/SME arm/grado cart.  Carefully inspected everything and got it working - really nice sound from a little system. SO we dont need new, and we dont always need fancy to enjoy music that sounds WAY better than most mass market crap.
Enjoy,
G

Well, these components have provided joy for 30 years. That's spanning the gulf war, the dotcom bust, the great recession to the current global pandemic. That's impressive. If you updated all of these components now, that should get you to age 80. Then, time to update again......

@cruxarche,   Kudos to you for acquiring a system like that at such a young age.  Until recently, I was using a Sony AV receiver, Jensen speakers and a Pioneer CD changer, all 25-30+ years old.  They still sounded pretty good to me, but at this advanced age (65) I decided to treat myself to entry level audiophile equipment.  My nephew now uses my old system and is happy with it.

Along with the advantages noted of newer equipment, I have seen people on this forum and elsewhere state that the quality of equipment has declined the past 20 years.  Unless you are paying big bucks, corners are cut to reach a price point.  For that reason, some people seek out vintage equipment and then have it checked by a qualified technician for recapping or what replacement is deemed necessary.  If, as you say, funds are limited, you might need to be frugal in trying to upgrade your sound (I have no idea what restoration of vintage equipment costs, but people do it.  I recommend videos by a guy on YouTube that does excellent work — xraytonyb.  Check it out).

It sounds to me as though you want a green light to spend money
again after your last investment 30 years back. 

If you follow these forums you must know you either care more about
money than SQ or you are just F-ing with us.

Fess up!!

Nice system and if you are happy then good on you. You also know that the equipment was cared for. If the capacitors are not swollen, or leaking you should be fine.

There is some good input concerning a good DAC and streaming. Consider looking in better source quality and music selection, first. Pardon, if I assumed you haven’t already as you didn’t mention your source material. Qobuz is a streaming service that provides a large selection of hi-rez music. A quality streamer such as a dedicated NUC computer running Roon (software musical interface) as it functions as a utility is economical, fairly east to set-up and works well IMO.  Happy listening.