They tend to sound a little harsh on the top, muddy on the bottom, and vague in the midrange. It's usually a good idea to replace them with polypropylenes, although in a guitar amp you should keep the paper caps around in case you don't like the change. These were designed as replacements for ceramic discs in bypass applications and tend to be quite compact. I do like them for bypassing, but polypropylene is better in circuits that actually handle audio. There are stacked polypropylenes on the market, but they're expensive and hard to find.
Polypropylene capacitors are a mainstay for audio upgrades.
Their quality is excellent, they're reliable, and there are now quite a few sources for them. You should avoid using metallized polypropylenes in tube circuits that place high DC voltages across them as they have a reputation for shorting out. Finally, polystyrene capacitors are the best you can buy easily. Mallory and Mial polystyrenes are available from Newark and other large distributors, and other polystyrenes are carried by Parts Connection and Welborne Labs. So one has to compromise.