want more bass

Systems

I have 4 10in. momos in a 91 chevy pick up(regular cab) with a JBL12001 running all for of them wired parallel. The box is individually seald chambered with at most .66 cu. ft. of airspace each. A pioneer DEHP9300 as a source. It sounds good, but I feel I should be getting more bass out of them. I've heard other systems with four 10's and you can't almost stand all the bass. What can I do to get even more bass at of them. I was thinking a X-over or maybe more amp power.
Any tips would be appreciated. Thanks