That HiFonics Series VIII Zeus is capable of drawing some serious current you will need a really stout 12V power inverter to run it. Trying to rig one yourself is really dangerous and you could end up damaging the amp or even worse your home! Radio Shack sells a 10A supply (around $70-$80) that will allow you to turn it up to some extent but for the money you spend on that you could by a cheap home receiver or power amplifier instead.
The Input sensitivity is to adjust the amp to the incoming preamp signal from your head unit or receiver. There is a button on the from to select the ranges, typically preout voltage for home components runs about 2V so I would leave the button "out" and the gain at about the halfway point.
You would be much better off using the Zeus in a car and getting a decent home amp, or you could sell the Zeus on Ebay since they still command $200 or more depending on condition.
Good luck