Oh this is a bit embarrassing: I leave the AMP-3 on all of the time and the AMP-2 was being triggered from my Proceed PRE. This accounted for some of the temperature difference. I've now had both amps on for the past 48 hours and there is still a difference between them - the 3 is almost too hot to keep your hand on it, but the 2 is "just warm". I'll experiment to see if there is any sound difference when driving the fronts with the hot amp-3.
Thanks again Duane.
Thanks again Duane.