I have the 650s, the 599SEs, and I just finally bought the 600s - because they went on sale and I wanted to finally hear the differences for myself. To be honest, the 599SEs are damn good for the money. In fact, I did a headphone comparison with a buddy of mine (that's big into hi-fi and home theater but not into headphones at all) where I had him try about 6 pairs of my headphones. In doing direct comparisons between the 650 and 599se, having no idea what they cost, actually preferred the 599s over the 650s. The 650s warm sound takes some time getting used to them to appreciate their sound (in my opinion). For me it took getting a tube amp to sort of rediscover my 650s and appreciate them more. Something to consider.

I haven't done a good comparison to the 600s yet to form a solid opinion, but out of the box I think I like their sound signature better than the 650s too. But all this is very subjective of course. Seems like which one of the 6x0 series you like best is pretty much up to personal preference.