The Onkyo tend to run hot, but I haven't really heard of any overheating, as in having thermal protection kick in. Even my Integra (similar to Onkyo 905) seems to run pretty hot and it doesn't even have an amp section. The 906 is different than the other Onkyo models in that it has a torroidal power supply that makes it a stable, powerful amp. Most other receivers including the other Onkyo models to not have this feature.
But what is better about the 906.
145 wpc, 4 ohm certified amp, torroidal transformer,
Reon HQV video chip (I believe this still includes the MNR and BAR from the 905)
Upgraded DAC's, two HDMI outputs, HD radio.
Have a look at both and compare.
http://www.onkyo.ca/model.cfm?m=TX-NR906&class=Receiver&p=f
http://www.usa.denon.com/ProductDetails/3510.asp#