Yeah a good amp from the late 70s and 80s is likely still working to this day, and if it needs some TLC, you can surely fix them up. Should be nothing more than discrete components I would expect. Some big TO-3s on a heatsink and the like.
Though I have one of those little bookshelf systems that were prominent a few years ago. (Center amp section and two bookshelf speakers). I garbage picked an Aiwa system. Very nice indeed. Internerals were very good. A good indicator I like to look at is the electrolytic capacitors since they have distinguishable names on them. If they are a brand I've never heard of, I have to assume that they cheaped out on not only the elctrolytics but everything else. Likewise, if the electrolytics are a good brand Japanese capacitor or similar, then it's usually safe to assume that they didn't cut too many corners. This Aiwa system had nothing but Nichicon and Rubycons, and the good ones too, 105*C etc.
So there are some more recent amps out there too. Don't limit yourself to dinosaur amps. If you can take a look inside of one before you buy it, that would be nice.