Lifespan of amps and preamps?

Well, I was in a similar situation until last Fall (October 2019). Bottom line, I ended up upgrading important components and my sound is TONS better; I would encourage you to do the same.  I bought my stereo back in 1992 and did not change anything until recently. I thought it sounded great. Now -- I would never want to go back!  The only two components that stayed are my original KEF 104/2 speakers. What prompted me to do the upgrade was a series of small discoveries. It is an interesting story but for now I'll spare you the details. 
The first upgrade was interconnects (various Audioquest). I still remember my surprise at how much better everything sounded. It is one of these moments when you want to go though multiple favorite recordings to see how they sound now. And I was thinking -- "I had no idea how great these KEFs were able to sound all along and I just didn't know."  And: "It's the first time I am hearing this CD sound so great." Then you also know there's no going back.
So I remember being on vacation last September thinking how I want to go back home and listen to my upgraded stereo. But the real improvement came later. I started talking to a local audio shop (Q Audio in Cambridge, MA) and ended up replacing my Adcom amplifiers with Musical Fidelity M6si. No way I can come back to these Adcoms any more!  This time I spent 7 hours straight, listening to a lot of music to enjoy the difference (soundstage, dynamics, etc.) -- and again -- the same old speakers proved to have had the potential I never realized!
Last change was a new DAC: RME ADI-2 DAC FS. I stream into it from Node 2i (Qobuz and my CDs ripped to FLAC) + I am getting new life from my old (also 1992) Adcom CD player via digital coax cable from the CD to the DAC. So now the system is built around DAC -> balanced XLR -> MF M6si. And now I am at a point when I don't want to upgrade any more :-)
But the amp upgrade from the old Adcom was very much worth it.
I am encouraging you to do the same in spite of the wisdom in continuing to enjoy what you already enjoy. I wish I had done my upgrade sooner.

There is no hard and fast rule on longevity.  My system utilizes a midrange compression driver that is about 80 years old; it is one of the very best drivers that I've heard of any kind.  My amps, built about 8 years ago, have a lot of very old components in it; the output transformers are around 70 years old.

A friend recent bought an old Western electric 124 amp that was not reconditioned--all original parts, and the tubes, were also old and quite rare--WE 274, WE 348 and WE 350.  The amp is in perfect working order.  Western Electric built stuff for commercial use and with a cost-is-no-object philosophy; the company leased their gear so they built the stuff to last.

I appreciate all the input, as a newbie on this forum.  Thank you.  I will look into replacing caps, etc.

As I mentioned I just picked up a pair of SVS Prime Wireless speakers for my new home office.  It will be interesting to compare sound.  Even though it is a bit apples to oranges, the speakers are new tech, with good amplification and decent DAC. 

In a way it is kind of the opposite approach (amp, speaker, and DAC bundled), so i know its not audiophile.  But it is what I could afford. I cannot really build two systems right now.

I will test it out with Tidal MQA or other high quality recordings.  It will be my first serious comparison where I can really sit and listen for extended periods.
I will also take the advice of looking into local shops to hear new technology.  But I doubt that can happen for some time (COVID)...
Scott.

Regarding the question of maintenance on your NAD and Adcom both are due for a recapping. A general rule of thumb is that capacitors have a life of between 20 an 30 years, regardless of how often the components were powered up. These parts have a significant effect on the sound quality and the sound of older gear declines gradually.

I just recapped my 24 year old Krell preamp and even though it was working fine and sounded good, the new capacitors make a noticeable improvement in the sound.

The B&W's are a little different story. They should probably be recapped too but for some reason capacitors seem to last longer in speakers. If you can pull out and disconnect the crossovers the cost to put in new caps may be pretty reasonable.

The big question is if you have an attachment to this gear and want to keep it. It will cost at least a couple hundred bucks to recap each piece of electronics and if you're ready to move on you could take that money and put it toward newer gear. If you still like this preamp and amp (they are both great sounding classic pieces) then the cheapest strategy would be to get the maintenance done and hang on to them. But from a purely financial perspective it will cost more to refurbish them than they are worth. The third strategy is to just keep running them until they fail which could take years.

It's been my experience that you could spend a lot of money and get just a marginal improvement in sound compared to a recapped NAD and Adcom. I moved from an Adcom GFA 555 to a Krell KSA 300S the the level of sonic difference was disturbingly low compared to the difference in cost.

Most good equipment is rebuildable.  New Adcom equipment is cost effective.