There are two basic ways to achieve a low cutoff frequency in an enclosure: high moving mass/low compliance of suspension, and low moving mass/high compliance of suspension. Traditionally VMPS has fallen into the latter catefory. TC Sounds drivers and PR's fall into the former category.
Larger enclosures work best going the low mass/high compliance route, smaller ones favor high mass/low compliance
configurations. You can mix the two within limits. However, a Megawoofer (low moving mass, high compliance in a big box) usually won't push a TC PR *mass loaded to its limit) with sufficient force. The PR begins to resemble a solid baffle wall rather than a a compliant, pistonically moving diaphragm, and system F3 jumps higher, which is not what you want.
I have oversimplified greatly here, but that's the answer to your question.