Some people's ears are also broken. ;) Do you know how many people said the A900X and K550 sound similar? These two cans sound like night and day.
The highs on A900X are NOT harsh in the slightest. I actually get more sibilance issue with K550 than A900X. The A900X is sparkly without sibilance - it's just perfect. Part of the reason is because of how pronounced the mids are, as well as the upper-treble - it's balanced in such a way that they don't make the 7K-10K peak sound harsh. The K550 on the other hand rolled off upper-treble too early and the mids are too recessed in comparison, which causes a very harsh upper-mids/lower-treble, where the peak is (especially because you have to turn up the volume with the K550 due to thin and relatively recessed mids).
Sibilance occurs mostly in the lower/mid-treble; the delayed treble roll-off on the A900X does not cause sibilance, it simply brings out a lot more details relative to the K550 (whose overall treble presentation I consider to be on the soft side). I think instruments sound a lot more textured and natural on the A900X.
Anyway, people's tastes differ and I am not you - so if you are curious I would recommend you go listen to a pair and see for yourself.
Edited by Bagheera - 2/13/13 at 6:18pm