James,an amp that age likely needs a basic service,you should clean and retension the tube sockets,that is a likely cause of intermittent crackling and the like.Clean the pots,too.With filter caps that age there is no way to "test" them and definatively say they are okay.Manufacturers such as Sprague tell the retailers they supply that they have an unused shelf life of ten years.It is true there are people who leave them in for 20 or more years and have no problem,but if you ever have to clean up a chassis that had a cap explode and spew its guts you wont ever wait even the ten yrs.I change mine after about 8 years on all my own amps and I always notice an improvement in tone,particularly the bass response.And dont use the cheap Taiwan or Jap crap.Sprague and F&T are good choices.Russ,it is not a wives tale about not playing an amp causing the caps to deteriorate.If the amp is let sit without being fired up from time to time for years the electrolyte in the cap dries out and the first time they get hit with full voltage they will likely fail.That is why you form them even when you install new ones,and in an old amp it is advisable to bring the voltage up slow with a variac to re-form the caps before hitting it with full voltage.James,again I would suggest a good cleaning before looking for other sources of noise,it just might be all you need.You could spend a fortune in tubes when all it needed was a good cleaning.As for the caps,there are a lot of different opinions on when to change them,but if your caps are original,you cant go wrong if you replace them before they fail,and you will notice an improvement in the amps response.