How to make subwoofers hit harder?

I have 2 12" kicker comps they say 8 ohms on them. I'm running a 1200 watt 4 channel ssl amp. So I have them wired positive positive and negative negative all on channel 1 hoping that their going to hit harder. I see people with kicker comps that have theirs Windows flexing or mirrors shaking like no tomorrow, don't get me wrong these bitches kick, but I don't feel as if they have their full potential. I'm using 8 gauge power wire. 12 gauge speaker wire. Not sure about remote wire. The subs are in a Ford Explorer so it's not like
How in a car the subs take up the whole trunk. I just want to know how to make my subs hit harder and sound cleaner. Also have a factory head unit but don't think that makes a difference. So what can I do to make these hit better

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