Understanding input level on JL Audio 250/1???

Systems

I'm pretty much lost about the whole input level adjustments on my amp. I know that my head unit is sending a 4 volt signal but I am confused to what to set the amp to. I can switch between 200mv-2v range or an 800mv-8v range. You would think that I would use the higher voltage setting but it seems like the power output of my amp isn't as good. I also have a level knob to adjust as well. Will it hurt the amp if I have the wrong setting. Here is the link to the amp.

http://www.jlaudio.com/amps/2501.html

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you will probably want to put it on the 8 volt range and then adjust the level to 60% or about there.

that should match 4v to 4v.