In general a film cap will have a dissipation factor at least an order of magnitude, and often, many, lower than the best electrolytics, so at least in theory, they should be cleaner, clearer, or whatever you find happens with lower ESR caps.
As Desmond said, I have a couplepairs of blackgate NH seies non-polar caps that are 150 uF 350v, which I could, and probably will at least try as coupling caps in the crack. Not there yet though.
IMO you'll probably get more mileage of wrapping a piece of dynamat xtreme around the cap to help damp it.
Also what Desmond said about me and blackgates is not perfectly accurate -- in fact I fid most black gates to be over hyped, edgy, brittle and not worth the money -- with very few exceptions and in very few applications -- and the point is mostly moot as they are basically unobtainum these days. The big WKZ series power supply caps were superb, but even if you can find them these days, the prices will amaze you. FK and NH and N are really the only other BGs I've used but if a film cap that would have fit where I used them, I probably would have opted for that.
An all film cap power supply and film cathode bypasses on a SET amp is a thing of beauty, but the amp would have to be huge in order to accomodate the typical values/voltages we deal with.
I'm planning film PS bypasses in my stereomour, and cathode bypasses as well -- plus vcap copper teflon coupling caps (bought a used pair for super cheap) and a pair of 3.9 uF ampohm poly film/oil pf caps -- in case anybody wanted to know. If the copper vcaps turn out to be too much then Reality MRs will be subbed.
Anyway, go with the film caps, and where the silmics are nice as electrolytics go, they are not available in non-polar versions, so they can't be used as coupling caps in the Crack.
HTH,
Jim
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