sub is vibrating, not sounding good

Ok, so i have a rockford fosgate p3002 bridged and hooked up to an infinity perfect 10, when it hits it vibrates, and the sound of the vibration really makes the low frequency less ummmm distinguishable maybe. sub belonged to two brothers before me so im gonna buy a new one, but was wondering if this could have to do with the amp? vibration is not low pitched.and would it be better to hook it up to two subs or just bridge into one? sub suggestions in the range of 400 rms please.thanks

what does the vibration sound like? Is it stuff in your car rattling or is it coming from the sub? The sub could be blown, or you could have an air leak in the box.

the sound is coming from the sub, and there arent any air leaks. im gonna get either a new 10'' sub that handles 400rms or a couple 8" that handle 200rms what would you recommend. a ten would fit in the box i have sitting on the back seat of my truck, and a box with 2 8s would fit on the floorboard between front and back seat. what do you think of this offer http://www.sonicelectronix.com/item_1823.html

My guess is there is an airleak, its just coming from right where the sub is at.