I called Yamaha today and had a rather lengthy chat with the level 2 tec. All in all he said only way to get same sound at the same time out of the 673 is to put it in 7 channel stereo and use a switch for rear surrounds. Turn them on when you want music, or switch them to your main room when you want it to be 6/7 speakers. Not a huge problem and could even put an inline volume control in if needed.
The 773 he said gives me several more options I don't really need besides, party mode . ALSO it says PARTY on the remote, that's kinda cool. He said all things being equal there is not much of a bump in performance at the 673 until I move up to the A1020. Unfortunately that's out of my price range.
He told me that Costco has their own special version of the 773 called the HRT-7065. its the exact same receiver, it just doesn't come with the wireless adapter. Curious I called around to a few local Costco ( you wont find it listed on their online store, they have different products at the store location) and they quoted me 479.99 So, compared to the current amazon price of that 773($799) its a bargain.
But Newegg is having a sale on the 773 and its .....479.00 w free shipping. I paid $357.00 for my 673, cant decided if its worth the extra money to get the 773 if all I really care about is PartyMode. The five additional watts is negligible He even said, "its all marketing, no real reason besides that we don't include it on the other models". Really happy with their customer support BTW. Called this morning with my question and they gave me a ticket number telling me a lvl 2 tec would be following up and they called a few hours later. +1 for Yamaha.