Been doing a lot of A/Bing with ZST, ZSR, ZS6 and ZSN. So far, the ZSN is the best sounding from those four. It combines all the good traits from the other three models, but not only that, it also has the most forward mid and treble presentation than those other three.
-. ZST vs ZSN: - the ZST sound pretty subdued especially the high and mid (deep V shaped); treble is rolled off pretty quickly (cymbal sounds like SSss, very short decay); ZST has pretty punchy and good detail in bass, but it can, most of the time buried the vocal. Build quality and fit are good on both, but ZSN with its metal back is just gorgeous.
- the ZSN in comparison sounds airier, vocal and treble are more forward and you can easily hear what the singer say especially with song that has a lot of instruments going on, treble also more pronounced and decay is medium length (cymbal sounds like SSSsss); bass has same detail as ZST and with good amount of body and punch to it, but still just a touch lesser than the ZST. The ZSN is the better sounding IEM imho.
-. ZSR vs ZSN: - the ZSR has a little bit more forward mid and treble than ZST, but compared to the ZSN, the ZSR sounds pretty dark (deep V shaped); treble is quite airy and sparkle, but not quite forward. Bass is less punchy than ZSN and a bit more boomy. Again the bass can, most of the time, buried the vocal, but less than the ZST. Both ZST and ZSR sounds way darker and warmer than ZSN. Build quality is good, but fit is not that good as my ear start hurting after using the ZSR for about one hour or so.
- the ZSN, again, has more forward and airy in mid/treble presentation with punchier and more detailed bass. For me, the ZSN is the clear winner.
-. ZS6 vs ZSN: - the ZS6 has the most airiness and widest soundstage from all four IEMs, but its drawback is it is very prone to sibilance. The bass on ZS6 is quite punchy, but has less body than ZSN. Although ZS6 has more forward mid presentation than ZST/ZSR, it is still a bit behind than the ZSN. Build quality and fit are good, although you can over the time "feel" that it want to fall off due to its weight.
- the ZSN has less airiness and soundstage than ZS6, but with more forward in mid presentation, it sounds more intimate than the ZS6.
It is easy to pick the winner between ZSN and ZST/ZSR, but it is very hard to pick between ZS6 and ZSN. If I were new to KZ addiction, I would definitely buy both ZSN and ZS6. Hope that helps.