olegausany pointed out some important things to note. If you want a closed headphones (one where people can't hear your music, and your headphones block out some sound) the HE-400 are not a good choice. If you want a portable pair to go with you to a quiet environment, the HE-400 kill the D600.
Also, the D600 are now down to $310 on ebay / open box. Prices dropping on these.
what about the hd650's vs he-400 ?