I picked the add-a-trunk for 2 reasons. One, it's more secure than just putting amps under the rear seat (with modifications it's arguable). It's the steel add-a-trunk, and the way to get at everything is through the rear door. If it stays locked, everything is safe. With the amps I have, they are bolted down with hidden screws (see pictures link).

The 2nd reason was for basic weatherproofing. Carpet can soak up water and the seats themselves aren't waterproof (water can go straight through). The bottoms of the amps could be sitting in water should a rain storm come up and dump water quickly. On the back, inside of the add-a-trunk, it won't be a problem.

Pictures