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From what I remember, two speakers make about 6 db more than one speaker. If 3 db more is twice as loud as the previous sound level, that means two subs make more than twice as much sound, given equal amounts of power. Sealed boxes can be very nice, they don't have to be size matched to the same level as ported boxes. Ported boxes are more efficient, but the rolloff at the lowest bass levels is a lot higher. Remember to make sure the speaker is designed for a sealed or ported box, depending on which way you go. As far as size, larger speakers tend to be more efficient, need less power, and go lower. A 15" in a well-designed box can sound great, it does not have to boom. As far as taking the load off your front speakers, you will need a crossover to do that. If you have full-range inputs running to your mains, thats what you will get. So even if you have a sub with a built-in crossover, you will need another amp to power the fronts.
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