When you say "goes lower", elaborate on this. Lets take a certain song for example, will both subs hit the notes exactly the same (same amp and enclosure according to recommended)specifications)? Is the difference only in their potential? Meaning when the bass goes REALLY low, the 15 D2 ofcourse will have the extra capabilities of hittin' those extra low notes. If the difference can be distinguished in every song, then the difference is not just in potential. What I want to figure out is, is the 15" sorta like the carbon copy of 12" with the capabilities of going lower and louder? How will it perform on the songs that do not drop so low?