really depends on your price range.
i have sierra-1 lcr with 170se for surrounds.
i would of bought a sub from ascend but they are in development.
i ended up getting two mfw-15 from av123 for 1250 shipped.
The mfw-15 is a good sounding sub.
Some other good products many people on here reccommend are the svs, hsu, epik, and ed.
If price does not matter you can go with JL audio or Velodyne.
You need to be more specific as to your criteria for purchasing a subwoofer.