SDR = Super Dupont Resonator, it's a mod by a guy called sorrodje, you can read about it here:
I'd actually gotten mine from him, it tames the treble peak of the HD800 w/o affecting soundstage.
I've not personally heard the HD650, but from what I've gathered, the HD6XX sounds pretty damn close to it. So, if you wanna get a cheaper HD650, the HD6XX is it. The HE4XX is a pretty close match up to the HD6XX, but has a slightly more forward treble......I enjoy these two and they're my usual 'go to' cans.