I take it you're new to the modifying game? Did you literally just make a box big enough to fill the boot floor and cut a whole in the top? If so it will sound like feck all squared! Sub boxes need to be sized accordingly to the sub being fitted into them. Can't be too big or too small or else the sound will be awful, also the seams of the box need to be sealed with caulk to prevent any air escaping and it helps to fill the bix out with a cotton wool like stuffing to smooth out the sound. That amp is also going to overheat like mad too since it's jammed down the side of the box, there's not enough free air moving around it to discipate the heat it's going to generate so you'll probably find it'll cut out quite often