Other notable posts in this thread:
Link to measurements of another set of HD600 (my own):
http://www.changstar.com/index.php/topic,2432.msg68031.html#msg68031

Link to possible mods:
Mod #1: made by stratocaster
http://www.changstar.com/index.php/topic,2432.msg68458.html#msg68458

Mod #2: made by me (Bill-P)
http://www.changstar.com/index.php/topic,2432.msg68562.html#msg68562

Mod #3: some more additions by stratocaster
http://www.changstar.com/index.php/topic,2432.msg68617.html#msg68617

Contents of this original post:
This one is CEE TEE's pair. It's bone stock.

Super neutral, and maybe a bit hot at 3-4KHz, but I couldn't hear it on CEE TEE's amazing Moth prototype amp. Seriously, I loved that combo! It made the HD600 just ever so slightly less fast and euphonic than some Stax. That's an insane feat to achieve. Resolution is almost up there with HD800, and the only thing it was missing was the soundstage of the HD800 to me. Absolutely awesome little thing!

Channel matching is impeccable, and only offset a bit due to my poor positioning skills at low bass and >8KHz. I'd say... it's practically perfect by any means.

CSD shows extreme speed even down to 400Hz on my rig for both channels. Notice the extra decay is mostly due to enclosure/ear pads. In actual practice, I heard this headphone as being much faster than it is generally described. If it's a bit soft/warm/veiled, I think that's more due to rolled FR >8KHz and slight bass bump rather than to any resonance or decay. This is about as clean as anything can get IMO. Certainly much cleaner than some Stax (trust me, I still have measurements of the Stax SR-X Mk.3!), and some (if not most) planar magnetics. LCD-2 and Code-X actually don't measure this well on my rig. 

Side note: I created a new thread because I bought a HD600 as well. Will measure it and compare to this pair once it arrives. And then I may do some simple mods, but... honestly? If there is such a headphone that I don't mod at all, then this is it.