I received my ZS6 yesterday. Take the following with a pinch of salt though, explained later.
Using
http://www.audiocheck.net/soundtests_headphones.php I've checked the treble response and to my ears I didn't notice the spike. Also in just general listening my experience is that the treble matches fairly well with the mids and bass, with the bass seeming tighter than ZS5 v1. Mine were also in phase.
There is certainly more treble than I'd personally like but that is my preference as I'm a nightclub sound engineer so as you can imagine I tend to tune systems to get a great sub-bass sound then match the mids and tops to that (favourite systems by the way are Void ones).
Additionally I'm well aware that the upper end of my hearing is starting to go (I can't hear above 18k currently) so I have no idea whether this has affected my ability to hear the reported 10k spike, but for me it wasn't there. Maybe I got lucky or maybe my hearing is off