I got brief listen and AB ing the ZSX and V90 yesterday. Same as you, im sure these two on same level, I cant pick one if i only have to choose 1 of them. Both are amazing
Using same cable, same spiral dots eartips (which are i found the best for the two), same source.
Fit = both no problem with me, even bigger zsx made a good grip with my ears, but smaller V90 also get me instant fit and maybe better bet for most people.
Build = V90 because metal body that built like a tank, ZSX made by high quality plastic / resin with metal shell. Look wise you can choose by yourself.
Cable = default cable TRN one slightly better.
Comfort = V90, not by big margin though
isolation = V90, that big vent holes on shell plate just a gimmick, even if that thing is real, must be very tiny holes inside, i was hoping a semi open back one.
Bass = V90 definitely bigger on tuning, more to basshead level, ZSX lower in quantity that feels more balance in overall signature. Texture are good on both, maybe v90 just a hair thin better in texture. but quantity wise i prefer on ZSX level. Bottom end extension are great with the two, both are not rolled off too fast, on bass decay V90 have more slightly faster decay speed. Both have no mid bass bleed at all, just touch of warm and thickness, without sound muddy at all.
Mids = ZSX have more forwarded mids, but V90 perceives a clearer mids. Mids on V90 are more open, ZSX sounds a bit veiled on vocals, again not by much margin. lower mids on both have good thickness that not overly done. High mids on V90 are more emphasized than ZSX, made overall signature more dynamic, but on the other side can be shouty if you are sensitive on high mids, shouty female vocals on 3 khz, safe enough, but can be intense on high volumes, while ZSX more flatter, but for some people maybe sounds duller than V90.
Highs = V90 have crisp trebles, very crispy but not fatiguing, made the overal signature fun at first time you listening to them. They have smoother presentation yet very crispy and detailed. I like the trebles on V90 more than ZSX. ZSX trebles are more safe but trebleheads will want more engaging trebles. Both are safe from sibilance.
Overall signature = what differentiate one from another is = V90 more V shaped tuning, ZSX more balanced tuning. V90 more fun and engaging with bigger bass, ZSX more balanced and have forwarded mids
Detail retrieval = on ZSX micro details are more easily heard than V90, because of the more balanced tuning also.
Instrument separation = I have to give to ZSX because i heard more air in between instrument, quite good for this price range but V90 are not left behind.
Soundstage = They are both wide, but if i have to answer which one is wider, I will answer "maybe ZSX", again in very tiny gap between them.
Timbre = v90 is more natural to my ears, but remember ZSX isnt as steely as previous KZ. Just because of the timbre only, i would call its a big upgrade. they are closer to natural, but still below V90, again not by much.
They retail at almost same price also. So you cant go wrong with these 2, just about preferences. For me myself with my pop-jazz, pop, jass, classic, vocals, rock, blues, acoustics, I still cant decide clearly which one i would take to be frank. They have pros and cons but with only small margin. For tuning wise i would prefer ZSX, but i will miss that nice trebles on V90, and clearness of the mids. But if I take V90, i will keep thinking, errrghhh i wish the separation is like on ZSX and the bass is too overwhelming for me. That is the case if im allowed to be picky person. But at this price, if i grab randomly any of them, Ill be happy enough to listen without fear of damage or lost for daily use.
Now about BLON BL-03 that I received in between my ZSX and V90, I love the tuning more. The tuning the smoothness isnt from budget level. Its a tuning similar from what I've heard in TOTL level. crazy in this price. resolution and clearness also very very great, trump both ZSX and V90. Overall more dynamic, clear, with very natural timbre. On separation though, BLON lost to both ZSX and V90, multi driver iems have their advantage in this section. But as you can read in many reviews, blon fit are infamous, sh**ty pieces. That is the main cons of BLON.