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It's more of a tone thing. The 600 is technically more flat but to my ears has a lack of natural timbre that the 650 has. The 650 has a little more in the bass, which make it sound warmer. Most audiophiles are people who dispise bass and live for treble, which are the people who review the 600's as better. Nothing wrong with those people, but they are more plentiful in the audiophile world than the people who enjoy bass with their music. For enjoyment, the 650's are better to my ears.
I had to vote for the 598s, just because it's so hard to find good closed-back headphones that are suitable for commuting (where I listen to 80% of my music).
If Massdrop is teaming up with Sennheiser to bring back older models, then I'd like to see the return of the old HD590s.
They were my first pair of "proper" headphones, and although I've bought many other models and brands since (a lot of them far more expensive), none are as comfortable as those old HD590s. The weight, and clamping force were perfect!
They were probably one of the best I've had for gaming too, and I'd jump at the chance to have a new pair.