There is little in the $400 price range that is going to cause a marked improvement. Cheap subs do more things wrong than they do right. If you can pick up a Martin Logan Grotto or Dynamo you should be able to incorpotae it into the system well. The new NHT subs are not very good. THe older Sub2i was great as was the Sub2. Some of the Velodynes are a big bang for the buck. If you have a symetrical set-up stereo subs would be a nice thing to work towards.
Good luck!
Good luck!