Some glare and harshness around 800-1200hz - Does not make that much difference to the sibilance. - Pad mods fix this more than EQ.
I don't know why people complain that EQ has been talked about a lot on this thread, frankly it makes more difference to the sound then anything else you can do for the HE400, only other things that are talked about are amplifiers, pads etc. All of which have also been talked about a lot as well. This is the nature of a thread with 1015 pages..... But EQ will make the biggest difference with the HE400 as well as the pad mods.
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.
EQing and velour pads is the clear easy fix for the HE-400's for those trying to get the best possible sound with no major investments.
Pad Mod is, IMO, the only thing that saves the pleathers from the trash heap. They are just bad stock. TMRaven, where is the data showing frequency response with/without foam spacer rings? There seems to be some consensus that glare is an issue (but fixable with simple mods). I would have thought that improving the response of the 1 khz region would have had a positive impact on natural vocal sibilants.
I don't have the convenient link anymore, but I've heard some examples of slicing up vocals to support intelligibility / limit sibilance (by listening to the same recording over and over with narrowly defined bands), and, I can say, it is very much true that the midrange is a critical part (if not more), to controlling problems with sibilance. Maybe I'll find the link again sometime. But quick googling makes it obvious that recommendations for sibilance cutting (by people smarter than I) are all over the place - 1.5 khz - 7 khz and higher. The HE-400s scoop out many of the frequencies known to be sources of fatigue and sibilance, so I continue to find the disagreements here interesting. "Air" created by harmonics allowed to spill way into the high frequency range might be a convenient target, but I am flat not convinced its the only place one needs to look for sibilance problems.
Then again I've been clear that I pay attention to my recordings also, and can't fault the headphones for every negative listen I've had - some recordings just weren't mixed that great.