Dave,
We can take the max excursion limit example to the extreme say we want to drive 15mm excursion at 80hz. That is not practical because that amp can be 5,000W and it will destroy the driver in a second at anything lower than 80hz. The Max Excursion is really not just a number. A lot of IB and OB drivers are plagued with two things: 1) constant bottoming (at subsonic frequencies) and 2) no sufficient passband output. My explanation connected these two together. Actually it is quite easy to modify Siegfried's spreadsheet by adding correcting factor related to Q.
Steve,
I understand what you are saying. The 105db Danny mentioned was the peak from an actual music playing. It is not the max from 20hz single tone output. Although the front row position does mimic more like near field results. I calculate that four 12" in OB will have about the same output as one 12" in sealed with the same excursion limit, which is about 101db with 14mm of one way excursion at 20hz. What can be gained from the driver itself is the SW12-16 driver has a voice coil winding width of 2.2", about 0.6" longer than SW12-4. That translates to additional output. In short, Servo does not change the physical world. To take advantage, we need to think outside the box about what parameters are most helpful in squeezing out additional output. It is all very subtle, 20% here and 20% there....
BTW, one single A370 can drive up to 4 SW12-16. I haven't seen anyone going to that extreme. 
Brian
Danny, thanks. -but just want to clarify on my question, to be sure I understand though..
Would the 4 of the 16ohm ob sub drivers, now put into an IB sized enclosure, now equal 4 sealed 4ohm subs, or still just 1? -at 20hz
