I have heard both extensively, with systems built around them and (my multiple) recables to both.
I just sold HD 650 for HD 600.
HD 650 is the 'perfect' noobs can, it seems balanced, (especially to a newb) has the bass bloom, but incredible 'balance' that most newbs are looking for, (not hating, I was once there too) has pristine instrument separation, and made me hate my relatively high end (900$) surround sound system out of an Asus Xonar STX and a mid-tier offering Audioquest tos-link cable. (So really more like 1200$).
My favorite thing/s about the HD 650:
- Responds very well to a recable (as in it hangs with the big boys, and maybe lacks ambiance and a little detail)
- It's dynamic drivers are some of the best ever made and sound 'romantic' with the mids 'centric' nature of the can.
- It is a powerful movie companion (I mean it's the best movie headphone I've ever heard) and I never get used to how great it sounds for movies.
- It's the 'perfect' noobs phone as it has the perfect 'noob' signature and will grow with you as it did with me.
- i.e~It has the ability to scale well. (As in keep you away from the flagships if you're going through college or the like).
Personally, I've matured endlessly sound-wise over the last 3/4th year with this can and now prefer the accuracy of the almost dead neutral 'studio' HD 600. But I do miss the HD 650 (a little bit). Now I have my RastaPants 2 Fostex T50rp, which I (and the 4-5 others who have modded it to this configuration) think is more coherent, analytical, and detailed than lcd-2, rev 1 or 2, recabled or not. (While the Lcd-2 retains a bigger sound because it's driver are 4-5 times as large.
Edited by Hennyo - 10/12/11 at 10:14am