Will, you're correct, the ".1" channel in DVD-A can (and frequently does) have material over 80 Hz. The reason is that it's not necessarily a subwoofer channel, rather it's a sixth channel. The 950's BM strategy "converts" DVD-A for most home theater applications where the sixth channel is hooked to a subwoofer and the other speakers are not full range. The subwoofer channel gets the summed bass from the other five channels along with its own material and goes through a 120 Hz low pass filter. Anything in the sixth channel over 120 Hz is removed (I don't know what the slope is) and is lost.

In my setup, I'd rather lose the high frequency stuff than try to make my sub play full range. If you can listen to Fleetwood Mac: Rumors through a system without the filter and put a full range speaker in place of the sub, you'd hear early reflection vocals on a couple of songs.