Ok, I found the page where EML states all their warranty conditions -- so here it is, straight from the horse's mouth:
http://www.emissionlabs.com/html/guarantee/conditions.htmSee note 8 in regard to the PIO caution.
This said, I'm aware of many folks using EML tubes in various amps with Mundorf SIOs as coupling caps and have nevver had a problem -- perhaps due to the unusually high working voltage of 1200v, but whatever, in the real world this does not seem to be an issue with these caps. No, this doesn't mean I'm endorsing them, just as I'm not condemning them -- but just reporting what EML states.
I personally will probably not use any of my ampohm PIO caps in any amp where I plan to use EML tubes, but those are half the rated working voltage of the Mundorfs. Note, the Mundorf silver/gold/oil and silver/gold are metallized film types and are not a problem.
Hope this helps,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)