Well, my PB2000 arrived thursday, and I boxed up my new PB12-NSD and returned it. Although I unfortunately do not have any nice in room measurements to share, I can only give my subjective impressions:
Low frequency extension. I seem to notice a clear improvement in low frequency extension. The best way I had to test this was with the song "Bass I Love You". This song has a 17Hz frequency that I assume my PB12 was able to play, as I was able to feel a strong shaking/rumble in my room/couch at the end of the bass line, but that I could not hear. Playing this song with the PB2000 placed in the same location, the low frequency inaudible rumble lasted noticeably longer than with the PB12.
As far as sound goes, I have the impression that the PB2000 is punchier sounding in the mid and upper base.
Output, I was unable to notice a difference. Not that there is not one, I am sure there is. But in my room, the way I listen, I never taxed the PB12's maximum capability.
To test the PB2000's output, I ran it several dB hot and cranked my AVR to zero on electronica and I never saw any indication that the limiter was kicking in. This is much louder than I ever listen to things, so I am satisfied with it in my 2600 cu. ft room with several openings to other rooms.