I have the same enclosure n did the same but I just called kicker its under Warrenty (4months old) and sent it today thru usps priority box 21 dollars. the box its 12 X 12 X 6 exact fit for the 12 ill have it back in a week. Give em a call they might take that cut up one and tag this video
There could be 2 reasons, or too much power (which is unreal on music, only on sinus) and second is not much power and too much gain on amplifier (clipping). Also I saw that you have too thin cables to your sub. That cant be a reason but it's making your amp have less control on your driver, and making sub more smooth, the COMPRT very good sub, but it's already smooth, no need to make it more smoother.