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.
Preamp mods/Why capacitors?
I have a Threshold FET 2 preamp. Very nice, but clearly old - 15 years or so. It has been suggested that it could benefit from upgrading the capacitors. It seems that nearly all amp/preamp upgrades and tweeks I see involve the capacitors. Why is that? Are some just that much better? Do they tend to wear out over time? Is larger typicaly better? I'm not an engineer by training, but am curious about this.
Phil
Phil