Craig nailed the key details, but also keep this in mind... You don't want to "crank" your sub anyway. You want overhead so that you aren't near the limits all of the time.

Too many times people will buy a sub that honestly is too small, then they don't get the punch or the deep bass that they expect, and they end up with distortion as they try to crank it up.
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2-M60s, VP180, 8-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45, Anthem MRX-1120