We'll have to see the manual or wait until folks have units in their equipment racks to get all the details ferretted out - as the Model 950 proved, there's a lot that can go on with bass management.

The change in sub output between stereo and 5 stereo could be due to different crossover points on mains and on other speakers - if the mains are crossed over at 40Hz, most music sources aren't going to cross much over to the sub, whereas if the center or surrounds were crossed over at 80Hz or 100Hz you could easily see more data going to the sub once those channels kick in.