Since comparisons are on...
After a year and a half averaging something like 3 hours a day with the SE846 (80% of the time with the white filter), I'd say the Z5 sound better to my ears : wider soundstage and better depth too (Z5 has such a 3D layered sound), bass is more textured and refined (probably tied to better decay too) and overall has better impact (ok 16mm dynamic driver helps there) - sub bass remain the area where the SE846 is king, mids are probably the area where I think there is no clear winner although I feel Z5 have more body and I like vocals better, highs share a similar quality of high detail while being smooth (Z5 is smoother still and I like timbre better). The more I listen to the Z5, the more I feel the quality of the highs is overlooked, probably because it's so "civilized" it could use a tad more sparkle, but that's also what makes the Z5 non fatiguing.
Let me add a comparison with the IE800 I have been using a lot the past few months : soundstage is similar in terms of width but I feel the Z5 has more layers thus I perceive it as better sounding in that department, mids is where I'd give an edge to the IE800 I just feel they're more "lively" and "fun" although vocals are sometimes painfully transparent the Z5 has more civilized vocals which make them more intimate (a bit like the FAD Heaven IV), highs on the IE800 have more sparkle but can be fatiguing on some tracks/genre and sometimes they're too forward IMHO on that chapter the Z5 doesn't loose out on details but never is fatiguing. The IE800 are more transparent overall, never mind the source, the Z5 can range from lacking in that department to really good depending on the source (my own perception here).
Compared to the SE846 and IE800, there are things distinctive to the Z5 : the layered presentation of the sound is one very distinctive aspect which makes the soundstage as wide as it's deep (great note decay adds to that), never heard that in an IEM before (I have never tried customs and higher priced IEMs though) and it's very immersive. Imaging is revealed better somewhat. This makes all the other qualities of the Z5 more fun in my opinion. Add to this the coherence of the sound (something I liked in the single driver IE800 over the SE846) and you have something special (never tried phase coherent IEMs, and I know the Z5 are not but the multiple driver is nicely implemented here). Factor in the price and you come to realize Sony is bound to leave its mark!
One thing to keep in mind is the Z5 is better than you would give them credit for, both because your ear needs to adapt to the bass and also burn in is really a factor with the dynamic driver. I also feel more than the SE846 or IE800, the Z5 does benefit a great deal from a good source (I have spent quite some time out of PHA-3 balanced and I had some ah-ah "out of your head" moments with the soundstage and imaging). I'll definitely audition the Z5 in stores with high end players to check out what wonder can come out of them...
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Edit : I am past the 200h now and I am experiencing a whole new level of sound quality, everything just got better, soundstage even wider and deeper, bass more nuanced, highs more vivid... call this brain burn in or whatever, it happened also with the SE846 for me like something "pops" all of a sudden and falls into place. I am in awe with those IEMs, they're definitely something very special...