What is a HD33? Not familiar with that. Oh its an Optoma projector.
RX-V673 and A720 are pretty much the same receiver with the A720 about $100 more. Either one can stream music from iPhone, iPad or Android and there is also an app to control the receiver.
You get a one year extra warranty with the A720.
I don't know if there is a big difference in power from the 661 though. The 673 and A720 do not have 7.1 preouts to connect an external amp. The stop including those after the 667 receiver.
Doesn't look like your 661 can decode TrueHD or DTS-MA audio from blu ray but you may be able to send it from the player as multichannel PCM.