I'm sure there's a few things better about the HD-650, but to me it's just a different type of sound. I compared all 3 of them and the HD-598 is the most fun to listen to and the most engaging. It's like comparing the K501 to the K702. The K702 is "technically" better, but just not as fun to listen to as the K501/k601. I guess they call it less musical?
If you get the HD-650, expect less treble, less forward mids and less detail. Yes, I said it. HD-650 is quite forgiving of bad recordings with it's stock cable, but does improve with a better amp/dac of course.
Basically with the 598, 600 and 650 it all comes down to preferences. Everyone is going to say the one they like is the best
I will say that the HD-650 does have more bass than the HD-598/600.