I have a pair of Sennheiser HD580s (almost the same as the 600s) and their newer 595s. I really, really like both but give the edge to the 595s since they're a bit more forward with more defined bass (the 580s are a bit laid back). I thought about the 650s at one point, but decided I wanted to try a slightly different sound (the 650s are pretty much the natural evolution from the 580s, whereas Sennheiser went a slightly different route with the 595), and I wasn't disappointed.
The other nice thing is that the 595s are much easier to drive, and though they obviously benefit from a nice headphone amp (I use a great, cheap little tube amp called the
Little Dot II++), they sound decent enough out of your standard headphone jack (even the iPod can handle them well).
