With the expectations consumers would buy both or that everyone would be satisfied with one?
I may be naive, but I think Senn may be trying to make the best can they can (at a few price points and maybe more importantly, usage scenarios). Not the best, then one opposing. Using the HD650 as the pivot, going back in the HD580/600 direction (w/ K701, T1, etc. and general progress in mind).
That said I'm not expecting to like the HD700 direction from the HD800 perspective. I've mentioned my GS1000-like worries on the other site.