One that I remember hearing a lot in was Ben Harper - Diamonds on the Inside.
Actually, I've been listening to some more well recorded things, even classic rock such as some Zeppelin, and I'm gradually adjusting to the sound. Either that, or these demo cans just weren't burned in. The level of detail with these headphones is astounding. Almost doesn't seem real.
Edit: One more song: John Mayer - Waiting on the World to Change. Only a very slight sibilance tendency here, and a lot of the other tracks on the album. Not sure if it's the way his voice is, or was recorded. Didn't notice this on the HD650s, but we all know the HD650s pleasantly mask certain imperfections.
Sounds like you're just prone to hearing sibilance more than others. Same here. Quite a few members on this forum seem to not hear sibilance on tracks that I find so grating that I almost want to blow my brains out.
The T1 hurt my brain after a while. It's got one of the craziest treble peaks out there. Love the T1 bass though. I always go with solid state amps, but I feel that a little treble roll off may actually be a good idea on the T1. Tubes would likely do the trick.