to the original poster
you're thinking about this the wrong way..........its not about size.........u can't think that you're gonna get 15's just because they are bigger.........u have to think about what type of music you listen to.....the frequency range.....the trunk space.....that's how you decide what size speaker to get.....not just because 15's are bigger than 12's so than they must be louder. it's all about set up too....a good box and you have to have your levels and everything set right....it makes a huge difference of how the sub sounds