I can relate to your problem because I just purchased 2 12'' MTX 4000s and wondered how much they can pound. I noticed that they have quite a bit of excursion, and handle even the lowest notes. I strapped them in a sealed dual carpeted wedge box and hooked them up to a 450x2 amp and put them in my trunk firing into the seat so the wedge was 1'' parallel to the back of the seat. I'll tell you straight up, they ****ing POUND. They weighed in at an incredible 158 dB! But it all depends on where they are going, but that is was I would recommend.
The amp was a kicker 2-channel 900 watt bass amp, but jensens and kenwoods can throw em' down too.