I haven't heard the VTF-1, but I've measured my mirage *almost* true to specs down to 19 Hz (-4 db versus stated specs of -3). Pretty flat up to 80, where my cutoff is, as well. I love it. Current room is about 3000 cubic feet very open to an additional 2000 cubic feet.
With only one S-12, it was a nice addition. I added a second one recently due to the 5000 cubic foot total space, and it's unreal. I have them stacked, rear corner. Can't speak for the VTF-1, but heartily endorse the S-12. There's also a review out there by a "professional" reviewer on some audio mag or something that said the S-12 compared favorably to his VTF-3 (although lost in the end, as it should).