I have a Kicker o8zx400.1 Mono Subwoofer Amplifier, 400 watts RMS x 1 at 2 Ohms. I know your supposed to match the rms of the sub and the amp but which subs are worth getting thatll hit hard and "bump". Also what would be a good box for the sub. Thanks.

400 watts is really not that much power these days. Usually the best box for "bump" is a ported box.