could someone explain what all these are i know what they are but im still not sure which is best, i know it depends on what you are using but please explain. like ported are the ones with holes on the sides like tubes that go to the back of the sub, and a sealedhave no holes but do they have glass over the front of each sub? if not how good would that sound? not good i would think. and some subs only recomend sealed and no ported. ????????????? thanks alot