In your size room, here is what would be required for various playback levels:
Moderately Loud To Loud:
20-39PCi or PB12-NSD (equal performers)
Loud To Very Loud:
20-39PC-Plus or PB12-Plus (equal performers)
PB12-NSD/2
Very Loud to Professional Cinema (IMAX) Dolby Reference Level*:
PB12-Plus/2
dual PB12-NSD or dual 20-39PCi (equal performers)
dual 20-39PC-Plus or dual PB12-Plus (equal performers)
*Professional commercial cinemas (like an IMAX) play soundtracks at Dolby Reference Level, which requires clean/uncompressed bass peaks (not continuous levels) of ~117 dB at the listening position.
In comparing the PB12-Plus to the NSD/2:
The native extension of the PB12-Plus is 20 Hz with all three 3" ports open. In order to obtain 20 Hz extension from the NSD/2, you need to plug a port, leaving you with dual 3" ports open.
At 18-23 Hz (very deep), the PB12-Plus and the PB12-NSD/2 will actually have similar output capabilities (with the Plus being a bit higher, actually). Above 23 Hz, the NSD/2 will start to assert a progressively larger output advantage, worth about 50% (3.5 dB) more output than the PB12-Plus over the more common bass regions.
The PB12-Plus opens up the possibility of nicer finish options (for more $$), along with a PEQ control. The PEQ is a single-band parametric equalizer with three controls (frequency, cut, bandwidth) that allows the user to reduce the severity of a room-induced peak in the frequency response.
The PB12-NSD/2 is larger and heavier than the PB12-Plus, and only comes in textured black, and there is no PEQ option.
What I think would be best for your room and budget:
I think true Dolby Reference Level is a bit out of reach for your $1,000 budget. The PB12-NSD/2 in its native 25 Hz tune with all three 3" ports open and dual NSD woofers and 600 watts of power would come the closest over the most common bass regions. Tuned to 20 Hz (with a port plugged), it's still your strongest option at under $1,000, even while acknowledging but the PB12-Plus actually performs about the same or slightly better at the very deepest (18-23 Hz) frequencies due to its native 20 Hz tune with all three ports open.
If you want max SPL capability over the common bass regions, get the NSD/2. If you want extremely good efficiency and output at extremely deep frequencies, and can accept 3-4 dB less headroom over the mid-bass regions, then get the PB12-Plus along with the possibility of nicer finish options and the PEQ and a smaller cabinet.
If you were going to get the textured black on the PB12-Plus (forgoing the PEQ and nicer finish options), then the overall nod then swings to the NSD/2; you can't buy more overall performance for less than $1,000 delivered to your doorstep.
