Very Cool!! A new capacitor thread for the incoming New Year ( & I'm not being intentionally facetious ).
Todd, I'm curious, what sort of Motor-Run cap are you referring to ( Oil-Can as you call it ) ??
- PIO ( Paper in Oil ) or the more modern MPP in Oil ( Metallized PolyPropylene in Oil ) ??
To see if you can reduce the inherent HF hash that you're experiencing from the Audyn's Q4, give it some DC voltage for a day ( let it act as a battery for 24-48 hours so that the windings can swell and tighten-up somewhat ) >> use 18volts ( or more ) derived from some 9Volt batteries.
- Just make sure that you fully discharge that capacitor before putting it back into the HF circuit ( we don't want to hear about you damaging those nice AMT's ).
Tightening the capacitors windings can noticeably reduce some of the UHF hash ( also sparkle ) since these small inconsistencies in the winding tension can act like a mechanical generator for High-Q resonances
( offering an effect much like the Aphex Aural Exciter and how it was initially used in Studio Work starting way back in the late 1970's )
Just be aware, any capacitor ( that I've done this to ) never returns completely back to it's ( like-new, "fresh off the line" >> sparkly ) state .
Consider the DC treatment a form of accelerated capacitor "break-in" .
PS >> Happy New Year!!