12's are louder than 10's simply because they have more cone area, Back in the day larger speakers had weaker motor structures and less developed cones so the larger subs sounded like they were dragging compared to the smaller drivers in the same class with the same motor. Now, there is a mosconception that 15's and 18's cant keep up with some beats, which is incorrect. Ive heard a inhuman 18 with mirrored spiders and it was one of the best sounding subs ive ever heard.
Why not a single 15?, it takes amount the same amount of volume and would be crazy loud off that 1200.1.