Inputs 11-16 do not have a preamp. They just take whatever the line level is. You can turn those channels DOWN in the control panel on a computer. The switch on the back is just a small boost. So, you can use these inputs with the output of an electronic instrument like a synthesizer, or the output of an external preamp.
inputs 9-10 have a preamp suitable for a guitar pickup, but not enough gain for a mic.
inputs 1-8 have a full mic preamp.