The famous album S&M. It's a sibilance monster and it's no fun with the Heir. It's not a fault of the earphone but of the recording. Anyway, the Heir is too truthful to the recording. In comparison, the UE900 is pure music.
Another example (*ok, this is not sibilance, but definitely fatiguing): Adele - Cold Shoulder
Ouch! This one really hurts!
Edit: Don't get me wrong, I'm not saying the UE is better. I think the Heir is just more serious. It's the better choice for Classical, which should mean a lot. I think the Heir is where I want to be, but the UE is where I am. Maybe my opinion will change within the next couple of days.