Fantastic, detailed review. Thank you for taking the time to note these observations!
 
Have you tried D2000? Thinking about upgrading but I'm not sure.

I'd be interested as well.  I've tried the D2000s and was wondering how big of a jump up these really are.

Well I found the gap I feel is fairly large. They have a similar overall signature, but the th600 is more detailed, open, expensive, better sub bass quality and rumble, less sibilant and clearer all round. It isn't like 3x better or anything, but for me I would never be able to go back to a d2000 after this. You know that feeling once you've had it better.