ahhh, just to put my oar in. While I don't believe all the high end audio snake oil claims about this or that brand of capacitor. Going through a good quality amp and replacing the coupling/filter caps with polypropylene polystyrene types can make a subtle but audible difference even to the non "golden eared" set. Yes it's a lot of work and finding all the right values can be difficult and expensive. And the plastic film types tend to be 2-3 times larger, which can be a problem. Is it worth it? good question. If it ain't broke... If your amp is 20+ years old just doing a general replacement with new capacitors, fancy one's or not, will probably make a world of difference. Also if the rest of your system is not "up to snuff", the difference might not be noticeable. As a first experiment try replacing the capacitors in your speaker crossover. There size is usually not a problem and you can gang 'em up if needed to get the right value. If that makes a discernible difference to you then consider pressing on.
good luck and have fun, tomh