I started out life in Head-fi land with the HD580 (later added an OTL tube amp) and moved to the HD600 when balanced tube amps were beginning to be more affordable. The scalability of this series of headphones is really astonishing. In many ways this is mirrored with the HD800 with a practically unlimited ability to increase with gear capability.
I liked the section where you talked about build quality and listed the part numbers. I would submit that the broken headbands were a problem for the HD600 and HD650 as I still have my original HD600 and it's snapped headband from normal use. I bought a new one to replace it, thinking the headband was not available. I've recently been told the headband is available and it would be great to repair this to keep in my stable. Do you know the part number of the HD600 headband?