Let's make this clear: not many drivers are designed to have linear, low distortion motors. Most are designed to simply play 'loud', and at the expense or motor linearity at medium and higher excursion levels.
One solution to have a small size, yet very powerful subwoofer, is to use the Dayton 15" RS high fidelity driver. Use two per cabinet. In isobaric configuration(this means using the drivers, coupled in a short tunnel, one in front of the other, or by clam shell mounting them, front to front(but this is ugly - I encourage the short tunnel method. This will halve driver VAS, allowing a relatively small cabinet size. Using 4 cubic foot, with a slot port 11" wide x 3.5" high x 59" long, you will have a cabinet about the same total size as the 12" Kappa Perfect, but with even more SPL capability, and comparable SQ at all SPL levels. The Dayton RS HF subs have very high quality motors, making this an extremely high performance subwoofer system. These 15" drivers are roughly 160 each, I believe. So, $320 for the 'driver' to make one cabinet, or in quanity of 4, the price goes down a bit, to 150 per driver, so 600 dollars for drivers to make a 'pair' of the subs. The subs are 4 Ohms, so in pairs, you can wire them to be either 2 Ohms or 8 Ohms. Using the Behringer Ep2500/Ep4000, use in stereo mode, and you will have over 1000 clean watts per channel at 2 Ohm per channel. Plenty of power to knock dishes out of the kitchen cabinets.
-Chris