On the 8500, I listen at volumes that range from -20 to just above -10, probably averaging -15.5.
Given that, and info about your theater room from your other thread, I'd expect a pair of PB2000s should be adequate.
Circling back to your original question for a second though...
As I am upgrading a pair of Velodyne CHT-120s, the statements above got me to wonder if I should give consideration to evaluating a sub's capability to augment a soundtrack's +30 Hz content (in addition to a sub's <30 Hz ability).
To answer this, lets start by looking at a movie scene with some sub-30Hz content.
That is the spectral content of a short clip from the movie War Of The Worlds; time is represented along the vertical axis, while frequency goes along the horizontal, with the colors indicating intensity. There's a lot of content below 30Hz here...but simultaneously there's plenty of content above that mark as well. The complex nature of what we listen to, versus the simplicity of a CEA burst, is the key. A system with more output in the 30+Hz range is going to be using a smaller share of its resources (amplifier power for example) to reproduce that range for any given playback level, leaving more on hand for the subwoofer to simultaneously deliver the deep stuff.
