A couple of things. A ported sub only uses the backwave for a specific frequency band. It would be a complicated relationship, but only at probably 2 frequencies would you be able to claim that you had the displacement that was the sum of the
PR and active drivers displacements, that would be the frequencies at which you could simultaneously hit the xmax for both the active driver and the
PR. Of course, with your gain set like that you would likely destroy them at other frequencies, so it seems rather silly to add the two displacements. I'd rely on just the displacement of the active driver in the comparison and just note that you're going to get a low end boost from the
PR when comparing the two systems.
The four 18s will easily exceed the output of your two 15s in pretty much any conceivable alignment. If I were doing it though I'd probably try to rig up the 18s into a so-called structural LLT. In that case you're comparing apples and apples pretty much and the quad 18s happily obliterate the dual 15s in any comparison.
Which 18s are we talking about here?