This control determines what lower part of the frequency spectrum will be reproduced by the subwoofer and what higher parts will be handled by your main speaker. It is a "crossover" control. Rotating the knob sets the point where all lower frequencies will be handled by the subwoofer and all higher frequencies will be routed to
your main stereo speakers. As a starting point, set the control around 80 Hz. if you are using left and right tower speakers, 100 Hz. with bookshelf speakers, and 120 Hz. with small mini-speakers.
Note: If you use the Sub In connector, the Receiver Type toggle switch must be in the Pro Logic mode for the crossover frequency control to operate.