Great review. Great reading.
I have owned these atleast 3 times, and always really liked them.
They are really good headphones, but...
I don't feel, however, that they are dead neutral.
I have always heard a small annoying peak or emphasis around 3.5 k to 4 k that really bugged me sometimes. They could become too "hot" and had too much "bite", and could be somewhat "thin" sounding in this area on certain recordings.
The highs were also sometimes (rarely) a bit grainy, or a bit unrefined to me, especially compared to the HD 650.
Maybe it was my particular ears, but I always wound up selling them because of this.
I also felt they were lacking in bass extension and impact from about 40hz down.
Personally, I prefer the HD 650, X2, and HE-560, but thats just me and my opinion.
Again, excellent job on your review!
Glad you like the 600's.
You made an excellent choice!

Nice review. Opinion seems divided as to whether the HD600 is neutral. Most agree they are natural sounding and accurate. Some however see these headphones as having colouration and prefer the likes of Beyerdynamic's DT880 for neutral sound. The HD600s are great if you like intimate headphones and they are certainly more neutral than the HD650s.

Yeah I guess it has much to do with how everybody hears things and how you imagine things should sound.They are all pretty balanced headphones overall, but HD600s have something special to me.

I agree with Hal-X.  I also have the HD580s, but the HD600s sounds more relaxed and natural.  I am using a Cardas cable on mine though

are they any less sensitive than the HD580s?

It's the same exact transducer.