Following the "buy the most sub you can" credo, if you can even POSSIBLY stretch your budget $50, $800 will get you a Hsu VTF-3 MK3 (shipping included). If you could REALLY stretch, you could get an Epik Empire ($900 or so shipped), but that's quite a bit above budget.
If you can't stretch at all, I'd recommend the Hsu VTF-2 MK3 (around $600 shipped) or SVS PB12-NSD ($650 or so shipped). I've never dealt with Elemental Designs, but their return policy doesn't seem as generous (Hsu and SVS both give you around a month of in-home audition).
I don't think you can possibly go wrong with any of these choices.
My two cents.