As CB said, you don't need a ton of power to make a Mag 15 loud. That's actually one of the great things about them - they're not like most subs where it takes a LOT of power to get loud b/c they get plenty loud with equal to or less watts than rated power. However, 2 12's will be louder than a single 15 mainly due to their absolute power handling ability / beat-ability. To some it doesn't make sense, but it's the truth.