I am also surprised by how well mp3s sound. Just for fun I played a few songs out of a 1g iPod shuffle. I could tell everything was a little less precise but it still sounded about as good as listening to my old HD555s through it. What are you planning to get down the line?
After some careful listening I got a hypothesis for that phenomenon.
Although hd600 is hi-resolution but it plays low quality mp3s with correct tone color,
therefore restores the original musical feeling a lot and makes it sounds better than other HPs.