I found that a single 12" woofer is enough for me. It hits hard enough and doesn't overpower my mids and highs...
Setup includes a single dual 2 ohm voice coil Memphis 12" MOJO driven by the 1100 watt sub channel of a Memphis Belle amplifier. The woofer is in a custom built 1.5 cf enclosure with the amp rack built over it, then a steel grate covered with acoustic carpet hides the whole thing from view. I've had the amp for 3 or 4 years now, but the woofer is only a year old.
