I have two XXX brand 12" subwoofers with matching 1200 watt amp. How do I set it?

I start hearing distortion and you can't hear or feel the bass when you are outside the car, yet you can feel it some in the car. I was told the gain and bass boost needs adjusted, but I tried that and still can't get it to "bump" like I want it to. They worked great in my friends car, who I bought them from. How do I get it to bump so you can hear it from several blocks away?