I don't know anything about electronics, but I've been told that some of the capacitors available now are simply better than those manufactured in the past (e.g. 20 years ago). if that is indeed the case, it would make sense to upgrade older amps' caps, although that just raises the 'how much better will the sound be for the investment?' issue. I've upgraded caps before and noticed improvement which, while not mind-blowing, was clear. if you have a tech whom you trust and will do it on the cheap, it might be a low-cost way to get better sound and possibly extend the life of your amp too.
