The high level input are for hooking up a system that the head unit does not have preamp outputs. It's just a different way to hook up the amp. Most amps have high level input and low level preamp input. You hook the speakers wires to the high level input and you don't use patch cables. If you use patch cables you don't hook the high level input. As for the best way to hook it up I really don't know. I use the low level preamp input. My head unit did not have preamp outputs so I got a box to turn from high into low. Can't think what it's called right off.
It's a line-level converter..You are better off getting this part than using the high level in. a lot less distortion, less chance of engine noise, and easily interchangeable in the future. The list goes on..Chris