Well the 770 would probably be overkill (200 watts)for multiroom audio unless you really plan on cranking up the volume.
On the other hand when the upgrade bug bites (and it will) the 2 other channels will come in handy for 7.1 . Then you could buy a 2 channel for the multiroom later. A bonus is that you will need one less shelf initially.
You could save some money though with the 750 and a used 2 channel amp for around 200-300.
I know that I would be itching to try the 7.1 if my processor had it

Good luck