Get a pair of A.Bass Hawk 12s. my friend has a pair of 15s on a Cresendo Skyway 3k in 7 cuft @31 Htz and they Slamm!Nah... I have had them for a good long while. Though they still hit, it's time for some beefier subs.
CT Sounds Strato 10's
DD 700 series Redlines
or something in these ranges.