if your useing a nam brand speaker: kickers, fosgate, jbl,or whatever... there is a certain ammount of cubic feet your particular speaker needs to breath correctly in a sealed box...
now depending on the music style you listen to will greatly determine what type of box you want...
if you like RAP! eeewwwww then a ported box can give you the long low sustaining bass...
BUT if your a ROCKER like me then a sealed box will give you the punchy low rock bass and the mid to hi's that most rock bass has...
i think for 12's you need something like 2.0 - 2.5 cuft per speaker... and for 10's i think it is 1.5 - 2.0....
contact your speaker manufacturer or your local custom stereo store and im sure they can tell you what you need to know!
too much air = loose low bass
not enough air = pingy buzzy bass
and just the right ammount = hard stiff rock your *SS off blurr your eyes makes you dizzy bass...
hope that helps!
anthony