This is still semi-HD600 related -- the HD650s have more bass, noticeably more so, but are easily more colored. I really like the signature so it's okay, but the HD600s definitely win for a more balanced, albeit less "fun" presentation. That's not to say they aren't fun, they just aren't awesome at a whole lot of things. They're great with strings, cellos, that sort of thing. The resonance just comes out so natural, in my opinion. The HD650s aren't quite as good, as they are a bit warmer sounding. Vocals are definitely better on the HD650 but the HD600 is close. The HD600s definitely sound airier and not overbearing in any department. That's where they're great, is just sounding good with everything, and awesome with some things. The HD650 is way better at rap, though. Bass is awesome on these bad boys, sending a nice thump right into my ears. Voices are close and intimate on some recordings especially. To demonstrate:
The vocals are just so clear. Another great one from that album is
The Morning if you liked the one above. The first night I had the HD650s I listened to this album and I could've sworn they used the HD650 to master the album. It just sounds so great on them. They're not laggy at all on the Lyr. I heard them through my E17 when I was burning them in and the bass was a bit uncontrolled and slightly muddy. It's tight and punchy with the Lyr. Gotta love it!!!
All that said, the HD600 is still a great headphone, being much more balanced and still enjoyable with all genres. The HD650 has some magically intoxicating mids that make them better than the HD600 in my mind. It's kinda funny, the guy I got them from said he likes the HD650 more than the HD600 because he thought the HD600 had too much bass. Pffffft... the HD650 has so much mid bass, and it rocks! Violins/cellos on the HD600 are truly great though. Very enjoyable on the Lyr. It's a great amp, no lie...
When you get down to it, getting an HD600 new or like new for $300 is a decent deal compared to it being around $375-400 for a new/like new HD650. I don't think the HD600 is nearly as picky/hard to drive as the HD650 either, so you can consider it being $2-300 cheaper because you might not blow so much on an amp. If you like liquid smooth mids and really nice punchy bass, the HD650 is awesome. For a bit more open, less colored sound but still balanced and fun frequency response, the HD600 is great too.