Please help with my subwoofer [Kicker L7 15"]

[Mass Car Audio 10+ year member]

Try inverting your sub so you gain more internal box volume and see what that sounds like. Just remember to change the polarity on the output of the amplifier. positive to negative and negative to positive when the sub is inverted.

 

[GFuel 10+ year member]

that box looks way too small for the output you want at those low frequencies. no surprise imo..upgrade your box