double 18" reflex boxes question!

Hard to answer. To my knowledge, bassreflex box still can give most SPL for (small) volume once you look at power density. It needs beasty drivers though. In bassreflex box, you look for most displacement volume in the upper range, most motor force in the bassreflex tuning range, and biggest bassreflex ports for greatest efficiency due to the port compression. All balanced for best compromise. Bassreflex boxes are not exactly known for chest pounding bass, but it is doable with multiple boxes. They're known for "browner" bass, which might be more satisfactionary even at lower volumes.

The bass that moves your stomach is between 45-70Hz. For most accoustical pressure, you want to tune the box relatively high, at around 40Hz. But it is more complex than that.

Although PD.186 can sound like herd of stomping elephants, it is kind of outdated, and can be outclassed by many drivers with more cone  excursion and more motor force. I believe that PD.1850 is mightier driver in that regard, and if you want to save money, and the amp is potent enough, it will be better choice Than 186s. With stronger drivers, it is harder to push the required power into the speaker, as it has higher impedance across usable range. You need amplifier with greater voltage swing to be able to push enough power in there. You either save money, do what you want to do and be happy with the outcome in just new bassreflex boxes, which is not bad at all, or you choose completely different and more efficient approach with most efficient drivers, having greater displacemant volume, more motor force, etc. These beasts also need mostly different amps.