The hd600's are pretty boring in its un-amped form, even with a cheap amp like the pa2v2, it wont sound as good as you wish.
The Dt770s can be extremely much fun to use, especially if you like electro, dnb and\or power metal, and they unleash more of their potential with a low quality amp than the hd600's ever could.
But the hd600s are MUCH better all-arounders than the dt770s
Personally I find the 770s much funnier than the Hd600's, and with that setup, id buy the dt770s..
If you buy the dt770s you will sometimes really miss having another set of cans (depending on how often you listen to acustic music, classical, mainstream pop etc) but if you buy the sennheisers, you'll never be really happy with your music, cuz its a boring pair of cans, really boring.