Harman Kardon Citation Amp Owner's Manual page 36

Treble EQ
Move the slider to adjust the treble level of the left and right
speakers.
Subwoofer Crossover Setting
Select Crossover Mode to enable crossover setting. Move the
slider to set high frequencies you want to filter out.
Select Mono Full Range Out to output full range signal without
filtering.
NOTE:
• Crossover mode is only available when a wired or wireless
subwoofer is connected.
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