You cant go wrong with a IDQ man I have an 05 Silverado X-Cab also with a pre fab box and I have 2 Idq 10s and they sound good and I am really underpowering them and they still hit good. I have also heard the CVRs and those also sound good.
since you'll be having a mounting depth issue under that back seat in your prefab box (assuming 5.5" - 6" depth) and that your amp does the 400 @ 4ohm...you really can't beat the Dayton RSS265HO-4 10" Reference HO Subwoofer