https://wiki.fractalaudio.com/wiki/index.php?title=Connections_and_levels#Main_input_level
What is Input Level or Input Pad for?
Input Level and Input Pad are NOT GAIN controls. They do NOT affect the overall volume level, they have no effect on output clipping or on amp gain. The control only optimizes the signal-to-noise ratio of the analog-to-digital converters. The adjustment is applied before the A/D converter and is offset by a corresponding but opposite boost at the output of the converter.
Exception: Input Level below 5% on the Axe-Fx, FM9 or FM3 will have an impact on the signal level. Don't set it that low.
How to set Input Level or Input Pad
Make the red Input LED blink occasionally while strumming the strings. This is known as: "tickling the red". Make sure to strum hard, on the loudest pickup! There's a substantial range between orange and red. If you can't make the LED blink red at all, don't worry, it DOES NOT have an impact on tone or gain.
The red light turns on BEFORE the instrument input clips. It means "Warning, you're APPROACHING clipping" as opposed to "Warning, you ARE clipping". The red light turns on at -6dB from the point where the signal clips. While the word "clipping" is used here, in reality the input signal never clips, because of a limiter will kick in before that.
FM3 / AX8 / FX8: these units let you choose from fixed input padding settings. A higher Input Pad value means lower input level ("padding"), which increases noise floor. So keep the pad as close to 0dB as possible.