So I got my cable yesterday. It seemed to have tamed the hd800's treble and now I am back to using the neutral rings. There's a perfect balance in bass, mids, and treble now with the Zu cable and the neutral anax 2.0 rings.
The only true upgrade to the hd800s are the sr-009s, which will cost upwards of $10k for the headphones and amp. Even then, the hd800s will still excel in soundstage and imaging. Last night I listened to the live version of Hotel California from Hell Freezes Over and I was amazed by the hd800's imaging capabilities. It made me feel like I was part of the audience; I was able to pinpoint where each instrument was coming from, and plus I was able to feel that the audience was clapping right next to me as I listened.
That being said, the rest of the headphones in the $1000-$2000 range are really just side-grades, with different flavors. Sacrificing one strength for another's. I don't see myself wanting a HE-6, Audeze, or T1 any time over the HD800s.