CTK...

400 watts is plenty. The problem he's noticing is most likely a difference between sealed/ported and a box design that's based on Thiele small specs. Another issue is if your sub is being lost in the trunk. Fold down the seats and try firing in the cabin vs. towards the rear of the trunk.

One more thing...reverse polarity and see if maybe there's an issue with wave cancellation. Just reverse + and - wires on the speaker.