i read somewhere that the db difference between 12s and 15s was only 2 db. does that make a considerable difference of is the loudness of 12s and 15s not really that far apart. this is considering that they are the same kind of sub with the same xmax.
in real life experience yes ive seen a 1 to 2 db difference on average. A 1 db difference you wont hear; for me 2 db i can. If I was choosing a 12 from a 15 of the same brand and line id choose it from a musical point of view over a db point of view.