Ah, the ZS10 Pro. Insanely great IEM for the price point when it came out a year and a half ago. I seriously loved mine, but the ZAX is like a ZS10 Pro that does virtually everything better: better treble extension but less harshness and shoutiness, less bass* but more agile and textured bass, more detail and clarity* across the board, huge difference in soundstage, more precise imaging and layering...I love the ZS10 Pro and I think at the $25 price point they still slap, but the ZAX are straight up superior in every way I can think of. And more versatile because it's a less intense V shape.
*While the ZAX have less bass, and graph that way, due to vastly better instrument separation, bass guitars for example are much more perceptible and isolable, so while it's less bass, you actually hear that bass as something specific, whereas on the ZS10 Pro it's just mud. It's glorious hearing for the first time bass guitars on records where you've literally never noticed them before because they disappeared into the mix, and not only that but also hearing in precise detail the distortion of the amp, each fingerpick etc.
*Detail and clarity comes mostly, but not entirely, from the added upper treble, I think. But anyway, the ZAX are able to reveal many other things that less resolving KZs can't, all across the spectrum. Reverb on vocals that is otherwise hardly audible, defined scratchiness in vocals, etc. I'm amazed to think there are IEMs that are significantly more detailed and technical than even this.
That said, the ZS10 Pro do the "fun sound" really well. I happen to think the CCA C12 outdo them in the fun department along with being slightly more technical for a $3-4 price difference. But their V was too extreme for me and I find their mids thin. Still, fun they are.
ZS10 Pro are amazing for $25. ZAX for $49 are my favorite IEM purchase. Overall, I don't understand why some audiophiles take a condescending view of KZ. I think they make some of the best <$100 stuff for sure. Hoping the ASX is great now so that they can continue upmarket.