Lexicon I-O|FW810S Owner's Manual page 36

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8. Put on the headphones. While playing the guitar, gradually turn up the
Headphone Level knob until you reach a comfortable listening level. If
you would rather monitor the signal through the monitor speakers, turn
on the powered speakers or amplifier. While playing the guitar, gradually
turn up the Main Outputs Level knob until you reach the desired
listening level. Do not to turn it up to the point of feedback.
Main Outputs
Level Knob
Headphone
Level Knob