...i'm really in trouble with its trebles. I listened all style of Music form Infected Mushroom to Schubert and besides some jazz and some EDM I really can't live with these trebles . With he-400, my favourite Schubert piece (String quintet op 163) makes me grind my teeth.
I think we have the same experience and i have same feelings.
I'm using a Aune T1 Tube DAC/AMP ( Like RMac http://www.head-fi.org/t/604583/hifiman-he-400-impressions-and-discussion-thread/5700#post_9077560) and fortunately, it seems I found a tube which does a good job to tame these sibilant , sparkly trebles . IMO the problem is not only with trebles but with recessed mids. A bit lack of mids gives the focus on trebles ant that's hurting me. I tried to EQ my he-400 and the results is +2 for midrange and -2 from 8 KhZ to highest frequencies. Then I change the tube for a 6DJ8 instead of my 6N23p-EB .. and it made a kind of miracle.
Forwarder mids and a bit tamed trebles tend to eliminate sibilance in most cases. I listened to very different music yesterday and I think I could now keep my he-400 . Not sure yet but I think I can :)
I should be honest and say that it could only be a "brain and ears" burn but he-400 stays quite hard to listen from another source (my living room gear)
A week later: I Iike these headphones more and more. it seems my brain and ears need time to accustom to their signature. and Aune T1 + 6DJ8 Tube ( both Mullard and Amperex) seems to do the best job . Very strange experience in fact but I'm happy to be able to enjoy the he-400 now. I would have been so disapointed to really dislike the hifimans.
i was disappointed too when I listened to my Ultrasone SP for the first time but only few hours was necessary to understand the SP.