Too little airspace can cause the sub to not hit lower frequencies correctly. Some builders can "trick" the sub into thinking it's in a larger enclosure bu adding Poly-Fill which help slow down the soundwaves inside the box.
To be sure, Poly-fill can't make up for 1/2 a cubic foot of airspace, but it might be enought to help bring out certain frequencies that are bein attenuated by your current enclosure. You might consider trying the Poly-Fill if you already have the an enclosure built- the stuff is cheap to experiment with. Pick it up at Wal-Mart in the Arts/Crafts/Sewing section. It'll look impressive in the buggy next to your motor oil and shaving cream.
1999 Ranger 4.0 Sport --SOLD!--
2009 F250 XLT CrewCab ShortBed
5.4L / 6 Speed / 4.10LS