A question...

"Does the equal a priori rule only apply to classical systems, and not quantum ones?"

Actually, no! Everything we have been discussing in recent classes--the rule of equal a priori probabilities and the concepts of microstates, macrostates, and ensembles--are equally valid in quantum mechanics. It just turns out that the way you write these things out is a little bit more difficult to conceptualize since, instead of thinking of configurations as microstates, we have to start thinking of quantum-mechanical states. The classical viewpoint here will just make things easier to think about physically, but realize that it does not provide the basis for these things, which are instead very general.