Those are the optimum specs for external volume as listed in the manual. If its less i dont know what to tell ya.alpine recommended box is 1.3 cubes net //content.invisioncic.com/y282845/emoticons/uhoh.gif.c07307dd22ee7e63e22fc8e9c614d1fd.gif
EDIT: alpine recommends 0.75-1.0ft^3 after checking the maunal, 1.3 for 15".
Ive run them through winids and it seems that they are recommending the flatest response ported. But i hear ya there is louder out there.i'm sure...i think alpine just recommended that size so it will appeal to people who don't have much room...but it works best in a bigger size box...