I haven't heard the D2000, but I have the D600. The HE400 annihilates the D600 in may ways. The D600 sounds slightly "plastic" and a little sibilant to my ears. The comfort of D600 is top-notch, but I personally just can't live with them. I think D600 is really a bit downgraded to D2000. They now must be ended up for sale , so sad.
The D600 is very poor compared to the Dx000 denons.