I was able to hear a prototype of an STF at CES 2003 in January. I asked Dr. Hsu (yup -- he was there in person) what the difference was between that sub and the VTF-2. He said the STF was not tuneable, but the VTF was. Other than that, he told me there wasn't a real difference. (This is referring to the STF model that uses the same woofer size as the VTF-2.)