I realize this is a slightly old topic, but a I haven't posted in a while and I feel like typing this morning
Randomly, in my subjective
opinion, and experience:
Many years ago, the 1st really good capacitor I used was the Audio Cap Theta. There was no grit, no noise, and darn good dynamics. Wahooo
I used it subsequently behind some tweeters in a few projects, and in a coupling circuit (or perhaps two). Over time I experimented with other capacitors too... Mundorf Gold, Mundorf Silver/oil, Sonicap Gen 1 & 2, Sonicap Platinum, Audio Note (don't recall specifics), a few other "fancy" ones, and obviously, the "normal" Solen, Bennic, Axon variety. I eventually found the Theta to be slightly "dull" by comparison. Yes, it was good, but lacked sparkle - the exact sentiment you conveyed. I suppose that one persons "sparkle" is another person's grit & noise. Nonetheless, this is my opinion.
I think the Audio Cap Theta would be perfectly implemented behind good aluminum dome tweeter. I currently use Theta's behind some silk domes and it's almost too much "smooth".
During my initial years in hifi I was not a fan of bypassing, but eventually I did a good experiment with a friend and some good source gear. It was enlightening and changed my opinion completely. Generally, bypassing has an impact in ALL cases, but capacitor quality, character and implementation location MUST be considered. I do believe these are important.
Amplifier break-in does happen - especially if Black Gate capacitors are used. In my opinion, Black Gate capacitors are quite dull initially, but after 100 hours they come to life.
So... I do generally agree the Audio Cap Theta capacitor may be at the root of your perceived problem.
Specifically, I recommend replacing the Theta with a Sonic Cap Gen 1 having the same value. It WILL have more sparkle, but you will lose some of that smooth sound. If you generally like what you hear, install a 1/10th value SonicCap Platinum (bypass). If you can't get a Platinum, telephone Jeff/Elliot for their bypass recommendation. I suspect the answer may be a Mundorf Silver/Oil.
And... Great Project !!!