If you are interesting in modding or switching caps a good trial to do is to get low uF Audyn TrueCopper (0.1 or so) and bypass the output caps. Make sure to get the right V rating equal to or exceeding the existing (especially important in tubes). See if you hear a major difference. If so, you may want to go all copper foil but they are often very physically large.
Duroch Capacitors
Primaluna amplifiers and preamplifiers use DuRoch capacitors. Who makes them? What's their quality? I haven't been able to find anything online about this brand.