This should perhaps be in a different thread, I wanted to keep this one specifically about the Sonicap's construction/materials, but since we're already here..
Considering that PWK himself originally used military surplus oil-filled caps (yes I know milspec is usually quite high quality), I'd hazard a guess any modern cap will do. Actually, I've been wondering if modern caps might in fact have TOO low an esr compared to the old ones, I'm worried it might change the characteristics of the crossover. I recall reading something about "Q" factor in relation to ESR, and possibly even using a small resistor to make up for the lower ESR found in newer caps, compared to the old. Some of the theoretical stuff goes over my head. Maybe these factors are really too small to tell the difference.
At this point I'm leaning toward the ClarityCap SA's, because their manufacturer has been in business for 20+ yrs (it's good to support indepedent manufacturers who don't employ slave labor =) producing their own products rather than rebranding someone elses, they're designed in cooperation with well respected audio engineers (B&W among them, iirc) and they're realistically priced. PWK didn't seem too fussy about particular parts, so perhaps neither should we (my opinion of course :)
I guess since Sonicraft won't disclose the info I was looking for, we should consider this topic closed.