I have enjoyed my HD600 for over a year and just got the HD650. As other have stated HD600 are somewhat brighter whereas HD650 have more bass.
My first impressions:
- On some tracks you can't hear any difference or really barely but on others it's very apparent. Some music styles make it more ovious but the mastering seems to play its part.
- HD600 are more consistent, they deliver on any track
- HD650 can sometimes sound really better and sometimes much worse than HD600
- After a few hours it feels good to put the HD600 down to rest my ears whereas if it was not for the clamping I could listen to HD650 for much longer.
With blues or rock, the lead guitar has more bite on HD600 so that's what you tend to follow but it's more of an ensemble on HD650 so your focus shifts during the listening. Hard to say what I prefer here, I like the guitar more on HD600 but the balance is better on HD650.
Female voices can be too harsh with HD600 at times but otherwise I find both excel with voices.
With electronica and reggae it's a mixed bag. Mostly the extra bass brings the fullness that's often lacking with HD600 but on some recordings the HD650 "detaches" the bass from the rest and it's very detrimental to the balance of the track, making it sound unnatural.
With classical music I'll give a slight edge to HD650 on symphonies for the fuller approach but for chamber music/solos I prefer HD600, the bit of brightness injects more life to the recording. Opera voices are also more to my liking on HD600 as I enjoy them with more intensity. HD650 can be somewhat too mellow.
Honestly I like them both. A lot. Don't know which one I'm going to keep yet, it's a tough choice.