If you're willing to go a little higher....
Say, $500-600, then try an HSU or SVS sub. They can go much lower than most subs in their price-range. And they do so at very high volumes with very flat curves. After much research, I purchased the 16-46 from SVS and I use it with a surround-sound SACD setup. The bass, particularly for classical music is markedly better and for movies the chair moves over the floor if I turn it up. The only drawback is that it is a big unit and may not go with all decors out there, but I wasn't too concerned about that. SVS has a 30-day return policy, if you're not satisfied, but I doubt you will want to use it.