It really depends on what you like out of the ZS3 and what else do you desire out of the sound.
If you desire loads of detail with pretty good extension in the treble and bass thumping energy and a nice airy sound stage, the ZS6 is a good update.
But it has its issues for some people.
- Its fit can be an issue for some people compared to the ZS3.
- Isolation isn't as good.
- Its treble can be too hot for many people.
- In many processed vocal tracks, sibilance can be a major nuisance as its pretty prominent on the ZS6 compared to the ZS3.
The ZS6 is very good stuff like instrumental based music and maybe to some extent techno music ( if you can live with the treble that is.)
If you cannot live with more than 1 of the issues above, I would suggest the AS10. It basically has everything the ZS6 has without the issues. The only drawback I can think of which is subjective is the sound stage. The ZS6 does sound that bit more open comparatively. "Airier" , though I cannót honest suggest it to be " wider", its subjective based on how your ears hear them.
If you prefer something that has a much more "polite" treble, I would say the ZS10.
But it does have its issues too.
- Its harder to drive in that you need to crank up the volume a fair bit for it to sound the same as most other KZ's in terms of output volume.
- Its bulbous and short tubed, so fit may be an issue for some people and harder to keep in place and maintain a good seal as a result.
- Its sound isolation is pretty poor due to fit issues for me.
LoL ... if you like the looks of those, then perhaps you can take a look at the PHB-EM023