Anyone with a $400 budget should almost automatically choose the HSU STF-2.
It's $426 with shipping.
I have one and it is simply amazing. You will not be disappointed. If you look, the VTF-2 MK2 is their $500 sub, which has gotten many, many shining reviews saying it is THE best sub for the money. However, the ONLY difference between the VTF-2 and STF-2 is that the VTF-2 has 2 tuning modes (VT = Variable Tuning, ST = Single Tuning). The STF-2 simply has the lower extension tuning mode, which is what the reviewers used when benchmarking the VTF-2 and this is the mode most people would use anyways. So, theoretically speaking, the reviewers meant that the STF-2 was the best sub for the money.
HSU STF-2 all the way!