^ +1 to above but you can also exceed the manufacturers specs but you willhave to decrease power handling to comphensate for it.

A better aproach would be to build a bandpass box tuned to a lower frequency, remember to use(or enable) a subsonicfilter though or you will more than likely blow your sub(most class D amps have hte subwonic enabled by default - if you dont see it on the outside, open the case look for a switch of a button labled subsonic or check the manual. My polk 300.2 has no lable for it but inside the. Bandpass boxes sound better in open cab environments than sealed boxes.

Then more to think about, your 15" may just be at its limit. Theres only so much bass you can sueeze out of it. Two 12"s will give you more surace area than one 15.