Received the 600 yesterday and it feels like I have become overnight fan. They are just so nice. Not saying they are better or worse than the 650. What I am finding is that they do certain music better than the 650. And that's true also the other way around. My prob with the 650 is that it's a bit too bassy at times but worse, it lacks brightness and sparkle which can make them sound.. well, veiled and unengaging(?) At the same time, to me, the 600 does have that notch of brightness and sparkle but can lack that tiny last bit of bass that I like to get from it. Darn! Didn't see this coming. Expected to be sure to sell one of them again but they just complement one another. 600's for jazzy and stringy affairs and bassy recordings. The 650's for the brighter, bass-light and shabbier recordings. Something like that..
i found my peace in a hd650 + SPC cable .....it is my "hd600 on steroids" :P