Purrin's measurements reveal that the spacer for the pleather pads was a likely cause of the 1khz peak. One of the first things I noticed when switching from pleathers with spacer to velours more than a year ago was the 1khz region seemed less shouty.
Now then, as for sibilance, I get that you've been trying to argue about sibilance being a part of the vocal midrange, but sibilants are essentially to be treated as overshoots, and not as fundamental tones or harmonics. Their center of energy vary from around 4khz (f), 8khz (th) or even 10khz (s, t). People tend to complain about the s and t range with the HE-400. This is all naturally speaking as well. Hot recordings which tend to over-emphasize the treble region for the sake of false clarity make the th and s,t region even worse.
HE-400 also tends to have much less upper midrange compared to a lot of more neutral headphones on the market. They're relatively flat to around 1.5-2khz, but take a nosedive starting around 3khz. A headphone with much more upper midrange in comparison like the LCD2 ( a good 5-6db more in the 3-5khz range) is also way less sibilant in comparison, and that's because it doesn't have a Mount Everest centered around 10-12khz.
Edited by TMRaven - 12/15/13 at 8:48am