Ha, this is headfi, there are few here extremely satisfied with anything for very long. You seem to be an exception. The lack of LF impact was what drove me away from the HD800 but that was when I had the Concerto and vDac so am hoping they will do better with my current rig. I'm very interested in the HE500 and have updated my FS listing to reflect that I am open to trades (+cash) for either the HD800 or HE500 however the HE500 is pretty hot right now and I'll probably be hard pressed to snag a used one in as good of condition as my LCD-2's are but who knows. The things I've read about the HE500's rendering of vocals really peaked my interest. My favorite instrument is the female voice
I think the HD800 is a step above the HE500. When I upgraded, I did not really want to go back to the HE500's so I sold them and got HE6. And now it's HE6/HD800 depending on music.
The HE500 is fantastic for it's laid back sound, and woman vocals are probably a bit more natural than on the HD800. That being said, I think the HD800 does much more rights compared to the HE500 on the whole. It's just not as intimate.
The HE500 are still my #1 headphone for value, but if you have the money and amping requirements for the Senns, it's probably better.