I ended up with a cheap modification I'm happy with, it made the RCA:s sound a little more exciting. On an Essence ST it is(PCI), which has the jitter-reducing circuit missing on the STX(STXII has a better clock-chip instead).
I changed the I/V from 2xJRC2114DD, which is standard, to 2xLM4562NA which results in very good resolution and troublefree sound on the HP-out. LM4562NA has much higher specs than the 2114DD and that seems to work out nice on th I/V-stage. Also, LM4562NA are not very expensive.
After that I removed the standard 1x LM4562NA from the RCA-buffer and replaced it with one of the leftover 2114DD:s from the I/V. The 2114DD does a much better job on the buffer imho as it is more "musical" and full sounding in that position. The LM4562NA sounds sort of sterile, dry and a little lifeless - while reasonably correct.
It can be that it is de facto better to have a little lower specced OP on the buffer-position, as that position also functions as an lowpass-filter and too good specs may render the actual intended filtering worse. On the HP-out so does the HP-amp chip do the lowpass filtering instead, which means the LM4562NA:s or better does fine in that position(I/V).
Also, 2114DD is very proven as an output buffer chip for digital audio on merits of it's sound rather than it's specs, it is used on some vintage high-end CD-players in the same position. 2114DD is a development of the NE5532 which is a super standard chip but very proven and appreciated, 2114DD is sort of a NE5532 on steroids.
Anyway, this simple swapping of positions, at low cost, made my card sound considerably more nice and lifelike on both outputs.