I bought an 8 inch sub to put in the trunk of my '88 notch. It sounded good but I could only turn the amp up a 1/4 of the way without worrying about damaging the speaker. I decided to by another sub and hook it up through two channels and turn the amp up a little more. One speaker thumped harder than two, why is that? is there any way to make my two speakers (since i already have them both) thump harder without buying new equipment? Thank you.