I use a TC-3000/Audiopulse Axis in a 3cf, sealed box powered by a Behringer EP-2500. With a Behringer Feedback Destroyer as an EQ to lower the inductance hump at 55hz and boost the low end. If that is outside of your budget, then there are dozens of other subs. What is your budget because we can go from the Dayton Quatro to the Audiopulse LMS?
If you want to go with Audiopulse, do it quick. They went out of business recently and the drivers at Parts Express are just left overs from the last batch.
Well, that rules out the Audiopulse drivers, the Dayton Titanic drivers are $220. The cheapest 15" Audiopulse driver is the Audiopulse REVO 15" which is $510.
The Soundsplinter RL-p 15 dual 2 ohm. It has a low Qts of .333. I would want to the Vas to be closer to 140L like the dual 4 ohm version but 163L is still in the acceptable range. It will handle gobs of power so you can easily push it with a pro amp.