I received a recent ZS6, at first listening the trebles were rather calm, then appeared sibilance sometimes with more agressive highs.
After 80h burn in, I don't hear a lot off change. After 110h burn-in, trebles soften...
ZS6 maybe needs 150-200h burn-in
Dunno for sure but today I believe that the ZS6 "v2" was a false bet...
ZS6 is technical and the sound can be very impressive, but it does not forgive an average recording, or electronic.
I recommand for you the ZSR that is better tuned, the trebles are softer.
It has lot in commun with the ZS6
Everything is fine after 48h burn-in (loop of pink noise + sin log).
Voices are not so close as the ZS6, but not recessed.
Free Burn-in Files | Head-Fi.org:
https://www.head-fi.org/threads/free-burn-in-files.466827/
I bet that all earphones need a burn in to improve and get a stable sound. specialy for the BA that could need between 48-150h
Possibly KZ, to save costs (their products are sold at very low prices) does not any burn in, and we have to do the work.
I remember:
ZST pro, harsh trebles, recessed voices, well balanced after 70h burn in
ES3, sibilant all the time, artificial recessed voices, well improved after 80h