Onkyo used to run even warmer than Denon's.  Some of their current models still use linear power supplies like D&M's so if those still run much cooler, I would think it might be due to the difference in bias level.  If so then my guess is that D&M's may be biased higher, and would stay in class A a little more, like up to 0.1 to 0.5 W.   That would be a good thing for sound quality.    Just an educated guess though..