Yamaha vs Yamaha you say?
Also, don't assume that the higher end and more expensive models will sound better than the low and midrange ones. I owned and compared back to back a RX-663 vs a 3900 and their sound quality was identical. I actually preferred the 663 because the controls where more responsive. I think the main thing that will make one sound better than another is the progression in room correction programs (Audyssey, YPAO, etc...) And well, the 663 and 3900 where same year models so... Feature-wise, the 3900 was loaded but at the time I didn't care about bells and whistles, I just wanted the best sound for the price and the Yamaha 663 just seemed to be the best bang for the buck sound quality wise back when it came out (especially for those of us that where getting the AVS discounts). I would of kept the 3900 had there been a noticeable improvement in sound quality but it sounded just like the 663 but with laggier controls and a bunch of unneeded to me bells and whistles.
Now for movies, the Denon 3311 with Audyssey MultEQ XT, now that was a noticeable jump in sound quality, very noticeable in the bass and sub integration!

And the Audyssey curve is so smooth, easy, and non-fatiguing on the ears at reference volume while staying crystal clear!

(GUI was/is so much nicer than Yamaha too!) Yamaha is still my top two AVR choices though...