Assuming this thread is in reference to the lowpass frequency setting on the amplifier.....you are setting the crossover frequency for the lowpass crossover built into the amplifier.
The function of which I covered thoroughly in this post:
http://www.caraudio.com/forum/showpost.php?p=1746548&postcount=2
As to the original question....set it to where ever it sounds best. This is not something that can be answered over the internet, as the best setting is entirely vehicle, installation, equipment, and listener preference dependent. Listen to some various songs that you are used to, and change the frequency until you find the setting that best blends the subwoofer with the rest of the system and maximizes the sound quality of the subwoofer and the system overall. Typically it is somewhere between 50hz and 85hz...but
can be above or below this range depending on various factors. That range is simply a generalization.