I have a pair of 600s and a pair of Massdrop HD6xx (HD650 with cosmetic changes) and will compare them once they are very well broken in. I've heard them both but the 600s were very well broken in and the 650s were a demo pair at a place where they probably didn't see many demos.
The general take is that the 650s have a bit of a bass boost, but not distracting at all. The bass on the 600s is extended and very high quality in my opinion, but it's not prominent. Stuff like Daft Punk always seems to need a boost.
The 600s were criticized as too bright for many users so that was tamed just a pinch in the 650. Sennheiser continued to sell the 600s after the 650s were released and have sold plenty of each. The 650s are not a replacement or even an 'update' to the 600s. It's just a different tuning. Slightly less air/detail and slightly more bottom end. Both are plenty detailed for most folks.