If I didn't want the Gui and network connectivity I think I would be going with the 663, too good of a price point.
And if it can't handle 4 ohm speakers like the M80 you can still add a cheap 2 channel pro amp like the Behringer A500 for the fronts and be well ahead in $ spent.
Its really about the features.
Fred
------- Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!