That has more to do with how your sub is wired and what enclosure it's sitting in. To get the most power out of that amp, you should be wired in parallel (2ohms). Otherwise the most that sub will see out of the amp is 300W. I'm guessing it's the enclosure.
---------- Post added at 05:08 PM ---------- Previous post was at 05:07 PM ----------
Also check your HU and amp settings to make sure you're not double-filtering. LPF should be set to about 80Hz and make sure the sub level on the HU isn't all the way down.