I just sold my Nexus 6 for a Zenfone 2 because I wanted a dual sim phone (compatible with 4G EU bands) with decent performance and after checking out the OnePlus Two (crappy headphone out and sound BTW), the xPeria C5 Ultra Dual both of which I have tried (that last one has a nice headphone out, warm detailed and dynamic), I went for the Zenfone 2.
I didn't have any expectations but tried my with XBA-Z5 out of curiosity, at first I concur with what I have read here about bassy muddy sound but one thing to know is that the Zenfone 2 comes with a lot of bloatware including sound EQing that sucks and which is... enabled by default ! Turn that off and you'll get something very different that is fairly decent with a good volume level. I have not checked the audio spec or codec but my (subjective) impression is good soundstage and detail, and (yes) bass that are well defined and not overdone. There is good detail level, the sound is fairly neutral and I prefer some warmth but you can ditch this as crappy is my opinion.
I'll try out Zenfone 2 plus MDR-Z7, not sure it can drive those though, PHA-3 is soooo good,
Overall it might shock some but better output than the Nexus 6. Nowhere near any proper DAP or amp but in the top tier of smartphones and I have had a fair share of those geek that I am (S3, HTC One M7, Note 3, Note 4, Nexus 5, Nexus 6...).
I have read a few reviews that probably didn't bother to check EQ'ing was OFF, let me tell you it's not anywhere near the SQ you get once you've turned that ugly EQ off.