the vented box, if built properly will have a boost in the lower range closer to it's tuned to freq than the sealed box, which will be more flat, & not roll off until much higher than the vented box...
a port is a trade-off of gaining extra db, but loosing control below the tuned freq, & a steeper rolloff above it... it can be worth it if your goal is increased lower end output... which is why ported boxes are so popular...
average guy: bass+increased lower end output= goood
from a pure sq point of view, I personally prefer a sealed box in most situations... there will be no add'l boost in the lower end, but it will be tighter & more controlled... I like to be able to shoot for the real lowlows as much as the midlows, a ported box will give you a narrower band of higher output, & is pretty critical in needing the correct enclosure & port design if the final tune is off it can be less than great.....
if I choose a sealed enclosure, I choose speaker size dependant on the door speakers.. this is what I determine the subs on.....
if I have a 5 1/4 front door speaker, I would choose an 8" sub, maybe a 10" at the limit... the door speakers need to meld with the sub & if I went with a 15" sub my doors wouldn't play low enough to meet my upper range of the sub efficiently at all... this will cause a sloppy cross over point, & unecissary boosting in the xover range... you have to look at the rollof overlap & think about the xovers.... this is the primary mechanical tuning & the better you do efore the eq comes into the picture, the better...
a 6 1/2 is minimum to me for doors, & if there done nice & can provide a nice lower midrange, then the subs can be 10-12 no problem..., they'd have to hit nice & low to consider a 15 though.... I'd use a 15 with an 8" door speaker for sure.... the thing to think about is your first xover point to me when choosing the sub....
to me, if you really want the best bass, without any localization I say start with your front stage... the lower your front stage can play without rolling off, the lower the subs can roll off... if you use a good 8 & can get down below the 60's, then a ported box starts to look alot better, it'll be more efficent, & if the rolloff isn't going to cause any holes at the xover, then the only real tradeoff is having to have a steep rolloff below the tuned freq... where a sealed box can take advantage of a natural, much less steep lower rolloff, giving better reach into the really low freq. but not be as efficent...
in car natural bass boost is one thing, all vehicles gain will vary, but the xover freq is not really dependant on that, which is where the measured response comes into play... you can't really predict the natural cabin boost actual curve, but that's not important in selecting the right sub, sure it's a variable & a consideration, but not necissary in selecting a match in the doors to the subs..
hope this helps
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