For an additional $30, the Onkyo TX-SR606 outshined the RX-V663 on many levels in terms of both performance and features. Moreover, a week before I completed my evaluation, Denon announced a sub-$600 AVR with four HDMI inputs, a full suite of Audyssey technologies, and support for all the advanced surround codecs.
In addition, I have to give the Onkyo TX-SR606 a better grade on power output. It is rated at only 90Wpc compared to Yamaha's 95Wpc (though testing criteria can certainly affect the significance of this specification), but I was able to crank the Onkyo as loud as I could tolerate without any audible distortion or ear fatigue. Once I got a well-balanced frequency response on the Yamaha, the amp ran out of gas at higher listening volumes.