As mentioned, the HSU STF-2 is essentially the same as the VTF-1 when the VTF-1 is in "max extension" mode. The VTF-1 allows you to flip a switch and unplug a second port on the back so that it can play louder, but not as deep. The VTF-1 also has some extra bracing, which is why it is a bit heavier.
Personally, for your situation, I don't really see a reason to pay more for the VTF-1. Your room size is small, so extra output is not needed at all. And with 50% of your listening being movies, I think you're going to want that extra bit of extension vs. the extra output that you do not need. The bit of extra bracing in the VTF-1 is nice, but it's mostly there because of the second port (more holes means you need more bracing to make the cabinet stiff!). Honestly, the STF-2 is what I would choose in your situation. And I'm sure the little bit lower price and little bit lower weight are actually advantages in your case
The STF-2 does not lack any inputs that you would need. All you need is that lone RCA "LFE In" connection to go with the "Sub Out" connection on the back of your Marantz. The VTF-1 offers Left and Right inputs for old, old stereo receivers that did not have dedicated subwoofer outputs. But that is not the case here - you have a subwoofer output on your Marantz, so there are no worries about inputs
When I was talking about a "higher roll-off point", I was talking about the frequency at which the subwoofer starts to roll-off and get quieter. A "higher roll-off point" just means that the sub starts to get quieter at a higher frequency. So the Emotiva Ultra-12 plays nice and flat down to about 35-40Hz. Then it starts to get quieter and quieter the lower you go from there. On the other hand, the STF-2 plays nice and flat all the way down to about 28-30Hz, then it gets rapidly quieter below that.
With a sealed subwoofer, the only source of sound is the driver itself. When it pushes out, because the cabinet behind it is sealed, a vacuum forms inside the cabinet that wants to "suck" the driver back in. When the driver pulls in, the sealed air in the cabinet forms pressure that wants to "push" the driver back out. As a result, it gets harder and harder to move the driver further in and out, which is why the expensive, sealed subwoofers use massively powerful amps!
With a ported subwoofer, the air is free to move in and out of the cabinet, so no vacuum effect and no pressure effect to work against the movement of the driver. The port in the cabinet also lets the cabinet act rather like the hollow body of an acoustic guitar. The cabinet itself resonates and amplifies the sound out of the port through resonant reinforcement. The end result is that a ported subwoofer that has the same size driver and the same size amp as a sealed version can play lower and louder. But because the cabinet itself is making some of the sound, it isn't always a "tight" and "clean" as the sealed subwoofer. A well-designed ported subwoofer though can still sound "clean" and "tight" though, and the HSU, SVSound and Rythmik (just to name a few) ported subwoofers are well-designed ported subs
Anywho, yeah, when the shipped price of the Rythmik FV12 jumps up to $1200 or so, it makes it a lot harder to recommend against the fairly small sacrifice of just the deepest bass by going with the HSU STF-2! Heck, maybe you could get a pair of STF-2
As for setting up the purchase with HSU, just e-mail them. They are extremely helpful and friendly and they will set up everything you need in the way of shipping the sub to you, having the correct voltage set, and possibly even preparing special packaging if you need that for importing or tax declaration of what-have-you. E-mail them. That's the best way to proceed. I think you'll be very impressed with their service
