The crossover frequency of a subwoofer refers to the frequency at which the subwoofer separates the bass range of the audio signal from the higher frequency ranges. Correctly setting the crossover frequency can help improve the sound quality and balance of the audio signal. Below are some steps for setting the crossover frequency of a subwoofer:

First, make sure that the subwoofer is connected correctly and that all necessary settings have been made on the amplifier or receiver. The operating instructions for the unit should be checked to ensure that all necessary steps have been carried out.

The crossover frequency should be reset to the factory setting. If the subwoofer's factory settings are unknown, the owner's manual can be consulted.
A music source or film containing a mixture of high, mid and low frequencies should then be played. The subwoofer's volume control should be turned to a moderate level.

The crossover frequency should be changed slowly in both directions as the audio signal is played. Attention should be paid to changes in the sound quality.

If the bass range is perceived as too dominant or the sound quality is impaired, the crossover frequency can be increased to emphasise the higher frequency ranges more.
If the subwoofer seems too quiet or the low frequencies are not emphasised enough, the crossover frequency can be reduced to emphasise the bass range more.

Care should be taken not to set the crossover frequency too high, as this can result in an unnatural sound.

This process should be repeated until the ideal crossover frequency for the subwoofer is found. Patience and attention are important here to achieve better sound quality and balance of the audio signal.