On the topic of "upgrade from HP50", I think it's pretty clear to me...
Between myself and my Wife, we've got (among others) the HP50s, Senn HD-650s, HE-400i, and Oppo PM-3.
Now, I've never heard HD-600s, but I don't think that my 650s sound much like the HP50s. They're closer to the HE-400i, and yeah I think the 400i are a shade better than the 650s, or at least I prefer the less-laid back sound. I generally prefer the HP50s to either the 650s or the 400i, but the open cans are a nice change of pace from time to time. I wore open headphones for 10 years but in the past few years it's become clear to me that I favor the fullness of the closed sound over the soundstage space of open cans, especially since better closed cans can get pretty decent soundstages themselves with a good tube selection.
Which brings me to the PM-3. I've spent 90% of my headphone time in the past year with the HP50s, and with the PM-3s in hand, I'm really not sure I'll be putting the HP50s back on my head any time soon. The NADs are better un-ampd than the PM-3s, they do slightly better presence with vocals, and... well.. that's about it. The PM-3s are better at just about everything else. Considering how much I love the Barton twins (I've also got a pair of PSB M4U1 at work, that were my primary cans for a year and a half prior to the NADs), I was amazed that the PM-3 topped them pretty much across the board.
If you don't believe me, there's a PM-3 loaner program thread.