Each zone has a cross over filter that can allow only high frequencies, or only low frequencies to pass, or dsabled (and allow all frequencies to pass).
Both the crossover type and the crossover frequency has to be set before the crossover filter is enabled. Start by picking the type of filter and slope:
If you're unsure, use "4 - 24dB/oct Low Pass Filter" for the subwoofer zone and "5 - 24dB/oct High Pass Filter" for the main stereo speakers zone. The 12dB / 24dB is the sharpness of the filter, how fast it cuts off high or low frequencies. Experimenting with the setting and listening for what sound best, cannot hurt anything.
Now pick the crossover frequency:
This is another value that very much open to listening tests. Both the main speakers and the subwoofer should be set to the same frequency. When going up in frequency, this setting indicates the frequency the subwoofer stops working and the main speakers take over. This value is usually between 100Hz and 150Hz, but we allow values between 50 and 300Hz.
Note: Changes to this command should be backed up in case of power failure by going to Composer's Action Tab and pressing the "Backup Audio Settings (except volume)" button.