Before I can answer your question, it's important to know how your system is set up. What kind of equipment do you have driving the speakers, e.g. receiver or amp/preamp?
If you have a receiver, and the subwoofer is connected to the sub pre-out, then your receiver is handling the crossover and you should set the crossover switch on your sub to "out" to disable the internal crossover circuitry.
If you have an amp/premp, then you should adjust the crossover freqency knob a little at a time until you find the best blend. A SPL meter from Radio Shack can help immensely when determining where your frequency gaps are.
