For about $1500 or so, one can DIY a sealed and relatively small (5 cubic feet) unit using an Audio Pulse AXIS 15", Behringer Ep2500 amplifier, a DCX246 for FR compensation and xover and including wood/materials. It will have very high output down to about 12-13 Hz along with very very low distortion. Is 12Hz low enough? In an average size (2000 cubic foot) room, you should be able to achieve about 107-110 db at 12-14Hz, or 125-130db at 30-35Hz, with the above set up.
I don't know of a retail subwoofer, box store or I.D., that will match the DIY solution above even if you spend 2x the $$. At best, you might match it, given you spend enough, which will be a considerable amount. But I'm still not aware of a retail sub at any price that could match it, with the possible exception of the 2 x 13" JL Audio Gotham that costs a small fortune. Don't forget that with the true 2000 watt amplifier (EP2500), you can only use roughly 1/2 the linear excursion of the driver above. The amp will clip long before you begin to near the driver limitations. So, given an amp with 3x the power, you can still get another 4-5 dB(nearly 2x! the SPL!) of clean output.
-Chris