Yesterday and today I did some tests.
Certainly I can confirm now that the sibilance isn't present always. There is a tendency of the phones to push trebles really forward, but as there are a lot of tracks where I can hear sibilance and harshness, there are some others in which I don't have issues, and many others in which they touch the limit, but it's acceptable for not too much listening sessions. Generally I can find much more "S" or "F" or "TH" sibilance than cymbals harshness. The firsts are much more present, and I think that this is the only really issue that this phones have for me. I like anything of them, but this is a problem for me, they pierce my ears after a while.
I tried them with my Dacport, and the sound became more amazing than it was already, but any sign of loss of sibilance in tracks in which this was evident without the Dacport yet.
I got my hard drive in which I have my lossless collection, and had a confrontation with the same mp3 converted files that I have.
John Lee Hooker - The Healer - with Carlos Santana & Band, Flac: no sign of any harshness or sibilance, really enjoyable, with great soundstage and imaging, almost a perfect reproduction
Alter Bridge - Blackbird - Ties That Bind and Before Tomorrow Comes, Flac: as analytically I can hear anything and enjoy seriously this really congested album, I can analytically hear a quite amount of sibilance too.
Dire Straits - Six Blade knife, Flac: some harshness and some sibilance. Some harshness in guitar too.
Fela Anikulapo Kuti - Gentleman, Ogg: Sibilance in s, g, t, d. Too much for me. Trumpets are harsh too when go up.
I used VLC and Songbird.