Coupling Capacitor upgrade? Which one first?

Even I really doubt boutique cap manufacturers design their capacitors based on pure science..

Many "design" their caps just on the re-labeling of caps like Solen, get some of the data sheets and compare dimensions lead length ect ect of some of these boutique caps to Solen you’ll find identical data between the two. Just the price is 20 x different, just like what goes on with the boutique snake oil AC mains fuses.

Cheers George

Do you know anything about these brands and capacitors?

Audiocap rt or theta ppt
Jantzen alumen z cap
Rike audio s cap 2
Jupiter condenser beeswax ht or wax ht
Lefson c-mod
Intertechnik audyn tri reference
Clarity Cap cmr
Audience auricap xo
Jantzen superior z cap
Mundorf supreme evo oil

Solen is one of the largest cap manufacturers around, that do Hi-end audio caps.
I’m not going to name the brands that I strongly suspect of doing re-badgeing work, you’ll have to look for yourself on the data sheets and compare all the parameters, it will be obvious.

Cheers George