Hey everyone, I got the urge for another cap upgrade recently when I found a well priced pair of 800V 15uF Jantzen Superior Z-Caps on eBay. Installed them tonight and I'm noticing some really nice improvements over the Auricaps I had in there before (regular Auricaps, not XO as far as I know). The Jantzens, even without burn-in time, have a really nice attack to them that is a bit faster than the Auricaps which were really nice, but the Jantzens sound more transparent so far and I expect they'll only improve over time.
I'll provide another update after some further listening, but I can recommend these as a very affordable significant upgrade (just over $100 USD for the pair) for the Mainline coupling caps.