I like the Infinity Kappa 10 inch sub in a sealed box. One sub is all you will need in the small cabin of a crossfire. I also like Alpine amps. If your going to install an aftermarket head unit then you can use the front speaker outputs to feed the stock amp as it uses speaker level inputs. Then use the sub line level (rca) output if your new deck has one, Or if not use the rear line level to feed the aftermarket amp. Check out crutchfield.com for info.