I own an STF-2, but have never heard a VTF-2. IF (and that's an "if" in capitol letters) I understand things correctly, the "VT" in VTF-2 stands for "variable tuning." The VTF-2 has 2 ports in the back, and comes with a plug to block one of the ports. Having two ports allows you to choose between:
"Maximum Volume" -- With both ports open the sub can play as loud as it is capable of playing, but only reproduces bass frequencies down to 32 Hz.(It will reproduce some frequencies below 32 Hz., but at 32 Hz. the volume starts to "roll off," which means gradually reduce in volume.
or:
"Maximum Extension" (one port blocked - the sub doesn't play as loud, but reproduces frequencies down to the lowest point it can which is 25 Hz. (Again, it will reproduce frequencies lower than 25 Hz, but with gradually reduced volume as the frequency goes lower.
So, you can choose between having the sub play really loud, but not so low; or not quite as loud, but lower.
The STF-2 is comparable to having a VTF-2 permanently set in to "maximum extension." It has only one port, so you can't tune it. I don't listen at incredibly loud volume, so I didn't feel I needed the "variable" option. I did however, want those lower frequencies, so I got the STF-2. As I said, I've never heard a VTF-2, but, Trust me! The STF-2 plays PLENTY loud enough. If you want the variable feature, you pay $100 dollars more for it.
Hope this helps.
