Info from Hsu
Well, as we've received a post from a kind Hsu employee (thanks!), I have nothing to add personally but I thought I'd show you an email I just received today from another Hsu employee (I'm really impressed with this pre-sale customer service here!). Here it go:
Hello Jeff,
There is quite a bit of technical differences between
the STFs and the VTFs:
-For the VTFs, the bass characteristics can be varied
between lower extension(music mode) or higher
headroom(home theater mode).
-The VTFs have more amplifier power than the
STFs(hence more output).
-Between the STF-2 and the VTF-2, the bass extension
profiles are similar but since the VTF-2 has a more
powerful amplifier, it will product more output.
We will not make any MK versions for the STF series,
however, we are coming out soon with our newer
versions of the VTF, called HO(for higher output).
These HO versions will have higher headroom(i.e. amp
power of 350W and 500W) and a turbocharger accessory
which produces much lower bass down to 16HZ! They are
slated for release late Fall this year.