I can't speak for the 650, but will say this about the 600:
the 598 is snappier, quicker, more open sounding than the 600.
The 600 is, however, more refined, dense, atmospheric.
The 598 has better soundstage. But the 600 has superior harmonic complexity and doesn't sound as thin as the 598.
What I mean is, with the 598 you hear the meat of beats and sounds. With the 600, you get more of the complex air and atmosphere surrounding those beats and sounds. There's more of a dense cloud of ambiance surrounding the notes with the 600.
So the 600 is richer and not a "basic" sounding. One could argue, however, that the 600 can sound more congested in that sense.
The 598 has an edge in terms of timing and propulsiveness of beats and rhythms. The 600 doesn't sound as fast and organized in this regard.
Both phones sound great with the right amplification (the 598 sounds great with the Schiit Vali).
The 598 is more fun to listen to. The 600 is perhaps more emotionally involving.
Interestingly, since I got my HiFiMan HE-400 and HE-500, I don't reach for my 600s anymore. I find that HE-400 and 500 do everything I like in the 600 but do it one notch better.
I do still, however, reach for my 598s as they provide a different, more open, more fun sound.
598 are, of course, a better value. Especially if you get them on sale (I got mine for $149 at Fry's)
Edited by K.T. - 1/18/14 at 8:24am