axiom_man,
If you have a $4000 budget, I would suggest you look at a pair of SVS Ultra 13's, Seaton Submersive HP's, or Axiom EP800's. I hope to be looking for a pair of subs in that price range and those are what I would consider. Like already mentioned, check the audioholics review of the SVS, it is very through and makes me want to go get the SVS right now. The Seaton's haven't really been reviewed anywhere outside of forums, which is not enough for me to blindly purchase at that price point. Hopefully we can see a website review them at some point. Gene at audioholics requested one from Seaton for review and apparently never heard back. I'm not sure why the manufacturer would not want some publicity for the sub. Just by the specs alone, I don't see why it wouldn't do as good if not maybe even better than the SVS performance-wise. Many folks on forums have even posted graphs where the Seaton sub plays flat from the single digits up to 50 to 60 Hz (though again anything on a forum I'd take with a grain of salt). The Axiom sub has had several very good reviews as well. Though it is a little bit more expensive than the SVS. If you get one from the auction then it may be about the same price though.
