Kicker L3 10" Help

Hi People,

I'm bought a Kicker L3 4ohms and my amp is a GTA 1005. In the manual I found that the max rms for the amp is 400watts at 2ohms, my question is if I wire the sub in parallel to get 2ohms so I can match with the amp the equation is rms X 2ohms or 4ohms?. When I did the math using 2ohms I get 28.28v but I think there is something wrong cause it sound very low. Any help will be really appreciated.

Thanks