Preamp mods/Why capacitors?

They do wear out over time (some more than others), and yes...many people believe that some capacitors sound much better (or more transparent) than others (the same goes for resisitors, wire, and other parts too). Auricap, Hovland, and Black Gate are a few of those brands, but they're not cheap. Most equipment uses cheaper parts because they manufacture the piece to a specific price point...if they used expensive parts for everything they'd probably have to charge $10,000 retail. They're forced to compromise and capacitors are usually one of the areas where many manufacturers choose to cut cost.

Polarized capacitors will age and die but film caps usually do not. Yes there is a tremendous difference in sound quality between various types of capacitors, and the really good ones are very expensive, which is why they aren't usually used by manufacturers. So yes, you can sometimes get a lot of improvement by swapping them out. Stay with the same values. The ones to start with are the ones in the direct signal path- these will make the most difference.

...not only capacitors: fuses(especially on the output stage), internal wiring, resistors...
Capacitors will loose their dielectric abilities within the time and definitely need to be replaced.
For the best performance you shouldn't wait the end of their life and use even less than half(the same I would recommend for tubes).

Capacitors are very much like batteries in how they function and even in chemistry (i.e. electrolytic) so no wonder they wear out.
Some caps dry up over time especially if they get hot (i.e.Tantalum).

There as been a lot of improvement in capacitor technology in the past years,older designs did not benefit from those.

The funny thing is that your PC or cel. phone probably uses better components in some cases than your high end audio gear it's KHz vs. GHz , the good thing is those technologies eventually trickle down to Audio gear but rarely the opposite.

Dare to object Miket2us since to manufacture ...GHz capacitor is much simplier than ...KHz. Even two opposite leads in PC board can be used as ...GHz capacitor and participate in producing a carying high freequency. Moreover these are two completely different issues.