The NHT 1259 sounds pretty good, I used to own one, but it does not play very loud.

For true subwoofer duty, I personally recommend an Adire Audio Tempest from http://www.adireaudio.com . These drivers sound excellent in a 5 cuft box, with two 4" diameter ports 12" long, and they play extremely deep. I speak from first hand experience. They also don't require a lot of power. They will keep up with your front mains at pretty much any SPL level they can throw out, except at extremely high output.

Adam