Completely different sounds. The HD800s are transparent and completely neutral. I find them slightly bright where the treble can hurt a little bit. The new cables I got for them really compliment them well and I heard they benefit from the warmth of tube amps.
The Sennheiser HD800 is hardly neutral. The stock configuration without the foam mods is definitely a bright headphone with a fair bit of emphasis and ringing around the exact region sibilance occurs. Hence why a lot of people actually prefer the HD600 and HD650 over the HD800, despite the HD800 having some insanely low distortion, because the tonal balance on them is better and more flexible.
If Sennehesier bumped down the hump that occurs around 6-8kHz, it'd probably be perfect but that obviously hasn't happened judging by everyone getting amps, sources, and doing mods to reduce the problems around that frequency range. The HD800 is a perfect example of "buyer beware" because its one of the most technically proficient headphones in the world with some unfortunate problems that give lots of people buyer's remorse.