If you can't hear any problems, does it really matter

'Wearing out' problems would normally be electrolytic capacitors crying out, and are the sort of parts you'd 'blanket change' if you were restoring an old amp.

Obvious 'wearing out' problems would be noisy controls and switches.

Essentially transistors don't wear, but valves do, and should be replaced on a fairly regular schedule - which is difficult now with valves so expensive, and not as high a quality as the old Mullard ones (for example).

Yes as you say, the big problem seems usually to be the old paper wound electrolytics, but for many amplifiers, (particularly those for jukeboxes with which I'm more familiar) it's such a common problem you can get "kits."
However, as the 1980 Leak 2000 tuner/amp I bought three years ago off e-bay for thirty quid, which at the moment is behaving immaculately, if it goes "tits up" I'll just buy another, or something similar off eBay.

I think it's great these tuner/amps are now so "unfashionable"

I'm quite envious of stories I read on a USA board about acquisitions of Magnavox Astro-sonic phonographs, "Maggies" as they are affectionately known by enthusiasts of all ages who love them (what we would call radiograms), with their sophisticated amplifiers and multiple speakers, some models of which fifty years ago cost grandparents as much as a small car and have been kept in immaculate condition, are so unloved by their grandchildren who inherit them, they end up "cheap as chips" in thrift shops or even "free at the kerb."
Many are still working and often at most only need the amplifier re-capped and the turntable a de-gunk and "lube job."

http://www.youtube.com/watch?v=JmCxQ5mURmg