I have a Type S 12 inch in my 03 Ram now, however I need more. I want to build a fiberglass box apprx 2 cu ft for under my rear seat. Whats a better sub?

Alpine type X or should I go with a Kicker L7 ? Also will a 10 inch give me a tighter bass ? or am I good to stay with 12's ?

Rest of my system is all Alpine incl Lds.

Thanks