I just ordered one on tuesday, I suspect it will be here next week. I called hsu and asked the man there what the differences were between the two, he told me they both use the same woofer, cabinet design, and controls, only the STF-2 doesn't have the variable port and is locked in low extension mode. He also said the amp was a larger unit on the STF. I ordered one for 399 no shipping, saving $100 by not getting the variable port. The man also told me a STF-3 is basically a VTF-3 without the variable port, locked in max output mode. I have seen pics of the STF-2 back panel too, at this address.
http://members.iinet.net.au/~colinf/Newproduct.html
http://members.iinet.net.au/~colinf/Newproduct.html