I heard some treo setups when I was in texas. There pricey but sound clean.
They are also a supplier that you will never find at a bestbuy, circuit city, or car toys.

 

the treo is an amazing sub. the transient response is excellent and it has a huge low end. better than any of the ones listed above. Put it in as large of a box as you can and tune it low. RE Audio is a big dog in the super sub game. oh and dont be afraid of killing it with too much power.

 

I owned a couple of treo ssx's i think bout a year ago, sold em, but IMO there great subs, and whoever said there not really known for being nething but loud, I agree heh. But they can be made to sound pretty clear with the right box for sure. Great sub though.