Paxonator -> I bought theVS PB1000 about 6 months ago.
My listening room is large, with openings to my kitchen, hallway, and one to outside plus windows that are always open to the outdoors (3 of them). VERY hard room to pressurize.
I mainly watch movies, sometimes music say 75-25.
The response seems very flat! With more than enough volume for my playback levels. On batman returns, you can feel the impact on the opening scene when the bank manager shoots the shotgun, when the bus smashes through the wall, and many other scenes. The subwoofer can also pressurize the whole listening room. My place is all tile and concrete so nothing vibrates except the doors and windows.
If you were to run dual... I can't see them not providing enough bass... Or impact..
I'm no expert, but I would think 2 PB1000 would provide a lot more output in the 25-80hz range, and a bit more in the under 25hz range... I think it would be +4/+6 dB 25-80hz, and maybe +3dB 19-25hz.. Not fully sure, but I would think so.
I get these figures, because 1 vs. 1 they are almost the same in the 25-80hz.. so it would be almost double the output in that range with dual. Under 25hz, it is about a -3 dB gap... so it should be up about +3 dB with dual.. This is what I gather.
Because most of the "kick in the chest feeling" is from 40-80hz (again I think), the 2 PBs should provide a lot more of that feeling than a single NSD... Also you will get more wall shaking, floor vibrating, room pressure going on as you will be up +3dB in the ultra low... Again I think...
Like you mentioned, the response will be a lot smoother in all listening positions.
Not to mention, I feel less stress on each unit, and if by chance one blows (honestly I dont think possible with SVS) at least your only out half your bass? :)
Anyways, I'm just trying to help, let me know if you have questions regarding my setup, my PB1000 experience, and etc.
Hope this helps!