Peter,
Thanks for your input. I was looking at a pair of b-stock STF-2s, so that would run me closer to $650 before shipping. The idea of a single "super" sub like the VTF-3 would seem more palatable to me if I were able to place it in a corner or a nearfield position, however this is not possible due to room and WAF constraints.
Basically, my listening position is up against an exterior wall, about 12 feet from the mains which are in front of an interior wall that separates the living room from the kitchen and is open on 3 sides: at the top (18 foot vaulted ceiling), on the left (opens into the breakfast and kitchen) and on the right (opens into a hallway and stairs. To top it off it's a second story room so the floor offers little or no bass reinforcement at all.
In other words, the ROOM FROM HELL!!!
I once tried out a friend's Rocket UFW-10 sub for a few days and it was barely audible. With another friend's single STF-2 I had the gain cranked to 3 o'clock for movies and 1 o'clock for music and had to turn the sub at an angle so that the rear port would bounce off the interior wall towards my listening position. I know the VTF-3 will go lower and cleaner than the STF-2 but am a little concerned about getting sufficient SPL.