My Sennheiser HD558 arrived a couple of days ago, and I'm loving it so far. Really good sound, not fatiguing at all, something that made me sell my previous Gamer Headphone ( I know, very stupid ) but I'm noticing some High Treble Distortion. It's connected to the Headphone Output of the X-Fi Titanium HD, no Headphone Amp.
The example I have to tell you when I hear it is Led Zeppelin III - Since I've Been Loving you - MP3 320kbps. I have checked the file on Reaper, it's not clipping, well below the clipping zone. The distortion happens around 6:15 - 6:20. I don't know if it's supposed to sound like that, maybe because of the recording equipments back then, and the HD558 is making me notice it much more than before.