I checked this Yamaha's specs on a few websites and didn't see its THD%, isn't this a key spec when it comes to receivers?
No, humans typically can't hear even 1% THD let alone .08% THD and similar figures found on mass market gear.
Also in what regards is this Yamaha better than the Onkyo? Thanks for your time.
The Yamaha has multichannel preouts, the Onkyo does not. The Onkyo has been known to produce pops in the audio from time to time, this hasn't been a reported issue yet with the Yamaha RX-V663 to my knowledge. The Yamaha is likely to have a superior amplifier section than the Onkyo, if my theories about it using the same amplifier section as the critically acclaimed Yamaha RX-V659. The Onkyo's only advantage is the Faroudja DCDi scaler that can only be accessed through a hidden menu, most people wouldn't have much need for the scaler is it will likely compare to most TVs available now anyway in terms of scaling prowess.
