i think i narrowed my choices down im either going with 4 12 diamond d3 ported if can or 4 15 diamond d3 which would sound better or louder and if not how would you think 3 d3 12 would sound if ported and can some just give me what like the difference between 12'' subs and 15'' subs
diameter doesn't matter that much. Bl does. The Bl and Fs of the sub determine how tight and how deep it's going to sound. The Bl is your motor strength which tells you how well controlled the sub is, regardless of diameter. the Fs is resonant frequency. Tuning below this point in a ported box loses the benefits of porting. a larger sub usually has a lower Fs due to the moving mass increase, but you also have to use a loer low-pass filter on larger subs to avoid cone ripple which will make the subs sound muddy. If you go with fifteens, you may want to consider 8" midbass drivers to accompany them.
Well specs are a great place to start when looking at subs. But they can really only give you an idea of what a driver is capable of. The Bl and Fs are excellent things to look at, when purchasing a sub, but you really can't tell how a sub will sound, until you hook it up and listen to it.