LifelongCabooseThe HD600 and HD650 are pretty much the same headphone deliberately designed with different sound signatures.
I'm guessing that the manufacturing cost for either is pretty much the same. Since the HD650 is priced higher in the market, Sennheiser could offer a larger percentage discount to MD.
I prefer the more linear response (esp. midrange) of the 600 and have other headphones for when I want a more "V" type sound.
That said, if you own only one over ear open headphone, the HD6XX is a fine choice.