My perspective may be on the warmer end of this spectrum - I have a set of Pass XS150s, ~450lbs of all Class A in a room that measures 27X36X9ft. These idle around 700W. In the winter they can add 5-6 degrees to the room after about 4-6 hours of listening. In the summer, the air conditioning more than compensates - and the impact is nil.
I love the sound of Pass - especially Class A, and they have had some time to warm up. They sound even better after 2-3 glasses of amazing pinot, and even more so as the lights dim.

