If you have a subwoofer jack as your original post suggests, presumably your receiver or pre-pro has one or more crossover settings, e.g. it sends frequencies above 80 Hz to your main speakers and below 80 Hz to your sub. This is true even if you use separate amps, as I do. Typically, you specify this in the setup menu, often by classifying your main speakers as large or small. If using a sub, you want to select small, even though your main speakers are floor standers.
It's really quite simple; you are unlikely to need an external crossover filter to complicate things. I do recommend a sound pressure level meter, set to C weighting and slow trajectory. Radio Shack sells one very adequate for the application.