I share jerg's experience. I had the HE-500 before and the HE-400 felt rather unlike what I would expect from Hifiman. The first thing I noticed was bright spiky treble pronunciation near the region of a T90 (I had that tood for a while). I thought something was wrong with the headfone, it was more than surprising. The T90 can have a certain "zing" in the highs but it's still less metallic in the highs. The HE-400 has good voice reproduction compared to the T90 (whose bright character is also represented in the mids) thus the highs stand out more.
Leaving T90 or any Beyer out my HE-400 had too much fatigueing metallic treble. I really hope you take a look at this issue and maybe we can have a HE-400 again with HE-500 highs. :D