I have a ZAS and a ZAX (and had the ZS10 Pro which I didn't like much). The ZAX is way more balanced WITH good bass (but not as much of ZAS obviously). Really depends on what you listen to. I'm more rock/metal, so the ZAS's massive bass drowns out way too much of the frequencies that matter. The treble isn't too bad, but feels like it's missing something. I think it's the bass drowning things out. I'll probably still keep the ZAS, not bad at all.

I played Dr. Dre Chronic album on the ZAS and the bass was perfect. Seems the ZAS is better for hip-hop, electronics, bass heavy stuff. The ZAX is surprisingly perfect.

I'm kinda old, but I can still hear 16k, but nothing above. So I couldn't help you on the 16k+ range.

Also, I've been enjoying the Tin T3 Plus. Nice for what it is. Out of most of the things I've tried (not that many IEMs but enough to know) the KZ ZAX has been the most amazing.