My 876 sat on the top of an open AV stand, so ventilation wasn't an issue.
It did not shut down while driving my M80's up to ~100db (6+ ohm mode), but it did start to sound more stressed beyond that point. And by stressed, I mean that the bass lost definition, the highs became a bit screechy, and the whole soundstage became less coherent. Distortion without much added volume. Unimpressive.
At least when compared to how my LPA-1 handles the same situation, with SQ remaining clean and composed well past 100db.
Peter, You make a very good argument for you to bring in a 3808 to complete your very good AVR comparisons!
