The PPSL works just like a regular ported or sealed enclosure except for how the drivers are mounted which gives you the acoustic low pass and distortion cancellation.
I would probably go with about 10 cu ft per 21SW152 and a 10" dia 22" long port for a ~19hz tuning (or dual 2.5"x15" slot ports above and below the manifold). You will give up a little output around tuning to the larger box, but I think it will be more than adequate, especially with 4 drivers. It is already going to be a very large box. The slot manifold will waste about 3 cu ft. So an ~24cu ft box for a dual 21SW152. This could be done in your 24x28x72 space. It would be a big and heavy cab, but the sound quality would be off the charts.
Another possibility would be dual 21SW152s in ~13cu ft total with 4 AE 15" PRs. My models show a tuning around 17hz. You lose about 4-5db at 20hz, but will play a little lower and is much smaller. The PRs are $65ea and would be mounted on the sides opposing each other. The response is somewhere between the 20cu ft ported from above and a 15cuft sealed. It is about 4-5db stronger than a similar sized sealed box until about 17hz and eventually the sealed passes it by around 14hz. The PR
box could probably be made into a SH50 stand.
With a little EQ tweaking either of these will work great. The PR
version eliminates the backwave leakage, port resonance and port compression effects. It also allows you to add weight to the PR
and adjust the tuning down to the 13-14hz range. The PR
box is limited to the excursion of the PRs and the ported box is limited to the point where port noise becomes a factor. A bigger port could be used but that pushes the port resonance lower.
Either way, in half space, the dual 21 4 15" PR
is capable of ~116db at 20hz and the dual 21 ported is doing ~120db at 20hz. Both with 625w per driver. A single EP2500 for both boxes with 2 drivers parallel per channel would work, but more is better. The EP2500 would clip before the subs run out of steam, but your ears and house would like cry uncle before the EP2500. If I had to guess, you would need more than 500-1000w total for reference levels to 20hz.
Two of these boxes would provide enough SPL to 20hz for a small commercial theater. You could also substitute the TD18H and only lose 1-2db at the same power. There are two disadvantages to that. First, the TD18's will move more to achieve the same SPL. Second, the lower displacement TD18's won't work as well in sealed boxes if you want to try that route. The advantage to the TD18 is that it is likely the best SQ 18 on the planet.
Don't forget you will need a HP filter. A balanced MiniDSP would work great.
If this sounds interesting I can mock up a PPSL enclosure in sketchup.