There have been quite a few Onkyo 606 owners complaining about overheating issues and "popping noises"
The Onkyo TX-SR606 Owners Thread - AVS Forum
This is one reason why I went for the similar spec Denon 1909, plus it has better Audyssey features, such as Dynamic Volume, which if you enable it means you can hear the low volume dialogue clearly and the explosions don't blow the windows out (very handy for not annoying the neighbours / sleeping kids / wife etc). It also stops you having to turn the volume down when the loud adverts come on TV.
The Onkyo has an extra HDMI input (4 versus the Denon's 3)