A paragraph from the 68c manual:
Input Source Button. The first two input channels offer an Input Source button that
lets you select instrument or line level for the ΒΌ-inch inputs. When the button is
illuminated, the input will accept a line-level source, such as a synthesizer or guitar
amp modeler. Press this button to engage the instrument preamp when connecting
guitars or a passive bass.
I think the last sentence means to "press the line button" into the OFF position for passive instruments?
I think a guitar with a battery is considered active and would therefore need the line level input source selected?