I have played the STF-3 and VTF-3 side by side (I've had the STF-1, an STF-3, and two VTF-3's...all very nice audio pieces.) The VTF is virtually identical (perhaps a tad more oomph thanks to the 50 extra amps) to the STF in max output mode. It does go a wee bit deeper when in max extension mode, but doesn't quite have the visceral impact of the STF (or max output VTF mode) in movies when in this mode.
I've never heard an SVS sub, but hope to, someday. But I gotta tell ya....I have absolutely no reason to change out my Hsu's. I might add turbos at some point, but they're already knockin' down the plaster. These dudes
should break windows if cranked up, btw.