most systems, the sub will get set around 80 - 100 Hz, and that should be sufficient. If you are using vey small main speakers, you may want to set it a little higher.
Also keep in mind placement of the sub can make a difference.
The subwoofer i puchased is a KLH Signature Series, but it has a variable frequency knob, any suggestions on what should this be set to for my surround sound system. thanx Jon