I still say grab sub box first, because you cant buy any sub and plan to stick in a box under your seat, dang sure an ***, maybe even a Type R. Find a sub box that fits your truck and the space your willing to give up. Then look at it's mouting depth and airspace, and power handling. If it's like .66 to .88 per side, you cant get a sub that requires 1.50 cu ft of airspace to properly work.
So back to step one, but a box, then find its dims, and order subs, then get amp that matches those subs. (DVC 2's or DVC 4's, etc..)