I totally disagree. I got the KZ fever 1 year ago with the ZST, and since then I got the ZS5V1, the ZS3, the ES3, ZS6 (Black) and another ZS6 (Red - one month after the first). I must say that, in my opinion, the ZS6 is by far the better of the bunch. I like all when listened in isolation, but if I switch to the ZS6 I notice a large improvement in most areas when compared to the others (and I returned to the original large size tips after using foams and wide bore shallow tips for a while). Meanwhile I sold the ZS3 and the ES3 to colleagues (I have easy access to them if I need them for any comparison means).
To my ears the ZS6 looses in isolation to the ZS3 (by far) and in comfort to the ZST (not by much), but it's bette in probably every other area. The ZS5V1 are rather good with comply style foams, but (as some others have mentioned) sound congested with some tracks while sounding amazing with others, and vocals are kind of lifeless. The ES3 brings the life back to vocals but looses to the ZS5v1 in other areas. The ZS6 is at least as good in vocals as the ES3 and better in every other areas (and sounds coherent with most of the tracks when listening randomly unlike the ZS5v1). I really enjoy ZS6 bass, openness, sparkle, and timing. Yes, they might be a bit harsh in some higher treble areas, but either I am not sensitive to those frequencies, or my brain got used to it.
On a funny note, I just got the Tin Audio T2, which are meant to have lesser harsh treble, and yet I find them brighter than the ZS6, and even unbearably brighter with the included silicon tips (they are ok with the included foams, but still brighter than the ZS6
).
In what concerns the so called ZS5V1 sensitivity to the source (which has been raised by many to explain the differences in the perceived sound), I certainly agree with it, but on a funny way: they sound rather good from an iPad Pro; good from an LGV10 and from the Oneplus 5T, and bad from a Note3 and (surprisingly) from the iFi iDSD Nano.
These are my findings, and might not apply to everyone, but for me they are obviously good
By the way, this is my first post here, but I have been lurking around for a long while.