I would agree that the box is the most important part. By using one sub, you have twice the room to build a box, but in ideal situations two subs have more potential.
Also, x-max only really effects the SPL of a sub in a sealed box. In a ported box, x-max means very little, and then only near tuning frequency. Specs are important, but x-max is generally not important in ported applications. Most loud subs have lower x-max, such as DD's and the above-mentioned strokers. The specs to look at for SPL in a ported box are mainly BL, Fs, Qes, and Qts, but it all depends on the application.

BTW, the brahma is more of an SQ woofer, and there are MANY subs that can get louder with less x-max.
Most high x-max woofer are for sq (W7, tumult, XXX).
I would take the single larger sub personally because you have more freedom in box design (unless size is not a problem) and it is always cool to make a lot of noise with only one sub.

 

Oh and outside of a sealed box, I seriously doubt that a brahma would embarass an L7. Kicker has done a very good job with designing a loud sub without following the industry standard of making high-x-max subs.

 

I have heard the L7 in a SQ pro vehicle, it did VERY well, and it blended very nicey with the all Alpine setup. Never be surprised how good a system can sound with the 'wrong' components and how bad 'good'componentscan sound. In SQ and SPL the expeienced installer is your best friend