As mentioned by Jallen, be careful with PIO caps in a hot tube amp chassis. Most PIOs are rated to 65C(and some optimistically so), as compared to 85C or above for dry constructions. ClarityCap MR has a dry construction and is treated with encapsulation and several other techniques that reduce microphonics for the effect of smoothness & relaxation similar to oil, but without the blurring that typically accompanies oil. The main problem has been unavailability of MR in the .1uf-.68uf range applicable to most tube applications. These small values are now available on an OEM basis and may be forthcoming through the retail channel.
Personally I'm not a big fan of Russian K40Y9 for signal coupling, but I like Russian SSG silver mica for small bypass applications. IMO these are more refined in treble than the Russian FT teflon series. If you can get by with 350Vdc, you might try paralleling a .01uf/350V SSG with your chosen .47uf base cap. The mil spec russians are typically conservatively rated for voltage so you should likely be fine.
Dave
ClarityCap North American OEM Sales Agency
Personally I'm not a big fan of Russian K40Y9 for signal coupling, but I like Russian SSG silver mica for small bypass applications. IMO these are more refined in treble than the Russian FT teflon series. If you can get by with 350Vdc, you might try paralleling a .01uf/350V SSG with your chosen .47uf base cap. The mil spec russians are typically conservatively rated for voltage so you should likely be fine.
Dave
ClarityCap North American OEM Sales Agency