Interesting. I had a look at their site, and there's an interface box between the audio out and the transducers - does it encode it somehow? Or does it connect via USB or something? I could only find the audio box version online. I've been considering a butt-kicker anyway, but simvibe looks like a better solution (provided I have somewhere to mount it).
(sorry about the off topic).
Simvibe is only software. You still need buttkickers/transducers. Since simvibe has to formulate low frequency sound to send to the transducers using the in game telemetry, it has to use a second sound card (I use my 7.1 on board for sound and a 5.1 pcie for simvibe). each output of the second sound card can support 2 transducers (stereo sound channels).
Now you can buy and use regular buttkickers to use with simvibe, or you can use a regular, multi channel av amp connected to multiple transducers.
I use a Yamaha htr 5640 and two 200w sinustec bass shakers which in total cost £160 ( same as a single buttkicker) and I can attach up to 6 more transducers at a cost of £30 each.
The "interface box" your seeing is the amplifier, buttkickers come with their own but they are pretty cheap and poor quality.