If you sound proof your trunk to eliminate all the rattles, come up with an appropriate box design, and correct crossover and eq settings, a lot of subwoofers out there will sound pretty nice.

By the way, have you considered going with a single high performance subwoofer instead? This could reduce the complexity and weight you carry around. One good subwoofer is Image Dynamics IDMAX. They just came up with the v4 version, which already show up online costing $400-500. However, the previous generation v3s are often available for a lot less. Should be a good buy for 200-250 for a used one. They handle up to 1000watts RMS power and the cone is replaceable.

Another "super subwoofer" is a JBL W12GTI. These have quite a following in car and home audio circles, but the power handling should also be outstanding. Just look at that massive basket.