More port area, right now you currently have 9-10" or Pa to Cube. Need between 12-16" on average. So pretty much make the dimensions of the port smaller, as its going to mostly free air the subs, but make it longer to gain more Port area per cubic feet.
Spl mainly revolves around three things; how you designed the system, how efficient the system is, and what amount of power you through at it. You need efficiency in order to put the power to use. I.e. a glass like finish inside the box, 45's placed in proper places, sanded corners, blah blah blah. The less effort the sub has to use to move air a higher score will produce.
Pretty isnt always loud.