I own a smaller version of that receiver -- the 773 is a great machine I wish I had it! NAD makes it very easy to use external amps or preamps.
On the back you'll find a series of RCA plugs labled PRE-OUT (L,R,C,SR,SL,etc), which have plugs jumping accross to another set of RCA jacks labled MAIN-IN (L,R,C,etc). For the channels you wish to amp seperately just pull that plug off the back of the receiver and connect (with good quality) RCA cables from the pre-out jack to your external amp. I've done this with my main L&R channels -- Preamp out to my external amp.
If you just want to use the amp section of your 773 because you have a good preamp/decoder you'd like to use then again -- same idea but this time pull the plugs from the back of the 773 and connect to 'MAIN-In' instead.