very simple only..seal your woofer box completely....
make sure there is no air coming out from the box when your woofer kicks hard....

there are 2 way you seal a woofer box
1)use silicon sealant or glue gun to seal ever corners of your box

2)use double sided tape and stick it on the back subwoofer speaker round frame(refer to the picture), stick into a compklete round shape..and then screw it back tighly into your box...this way you can minimize/seal the small gap in between you box and...