Best caps for updating my VTL 225 mono's ?

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

Changing capacitors to whatever brand is popular at the time is no guarantee you will end up with good sound. Many vintage components have been ruined by changing the caps to the latest greatest. Then there is the case of the reviewer who had his beloved Audio Research SP-15 recapped by GNSC. He was so disappointed in the sound he sold it at a discount just to be rid of it. If you are not familiar with the SP-15, it was one of Audio Research's top preamps and one of the best phone preamps ever.

mundorf makes the Mcap line and are far less money than the supreme line, but still a mundorf and is a very good cap, you should check them out.

I am assuming that the input caps are in parallel, and was done to decrease the input impedance or tailor the sound. Some would say that could "blur" the sound, others would claim that it would make it "faster". While I have not experienced these issues by paralleling caps, why not install one high quality cap that is the same uf value as the combine total of these input caps. The value needed is determined mostly by the input impedance and needed low freq roll off. Thus the needed value is the combined total of the paralleled original set. I have tube mono's that had input caps. I removed them after determining that the preamp had output caps, thus no need to have additional caps in the signal path!(only have two as of now) I believe the fewer the better not only for overall sound but a much easier time "tuning" the system with only two caps.

http://www.humblehomemadehifi.com/Cap.html

From our FWIW department.
Caps? You want caps? How about some major listening tests?