No, I think Atothex is right. It's not that those phones aren't great, it's that they scale up more powerfully than many others. Of course they sound fantastic, they're fantastic phones. But I think for the total price you pay, you'd get better sound quality with a more balanced system.
that seems to be the popular sentiment here but i have to disagree. i have never owned a headphone amp and i've been using HD600's for ten years and they sound amazing plugged into anything. in fact i just bought HD800s and when they arrive next week i'm going to see how they sound first before i spend another $300-$600 on an amp or dac.
everyone swears you need an amp but a lot of people also swear that cables matter and i've never seen a single double blind test that showed a sound difference in cabling. maybe i'm wrong about the whole amp thing and i'll probably end up getting an amp for the 800's just to satisfy my curiosity and then i'll know for sure whether or not the amp matters. but even if the amp improves the HD600, i think it's strange to think the 600's are a waste of money without an amp. compared to most headphones, they are a terrific upgrade.