make sure the box is air tight. Check your amps gains, eq settings etc...so you're not overdriving the subs.
Fibrefill or polyfill can be bought at wal-mart in the pillow section, or at any fabric store. It will virtually increase the size of your box by up to 30%, and tends to work better with boxes under 3 cuft. Add approximately 1 pound per cuft for a gain of abut 25% for that size box(I'm assuming it's around .5-1.0 cuft).

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2004 300M SPECIAL
Alpine CDA 9833
Alpine Pxa-h701
Alpine Rux-c701
Rockford fosgate p400-4
Rockford fosgate p4004
Hifonics BX 1205D
JBL c608 GTi
JBL p963
Infinity 2.5" center channel
Alpine type R 12"(2)
200 amp alternator
yellow top battery
0 awg power and ground (welding cable) 300 amp anl fuse
all active SQL set up with 2000 watts rms