When the sub has a crossover of about 120Hz or higher, it will generally be playing well up into the frequency range where you can localize it.
Are you using a home theater receiver? How is it connected? If so, you would set the sub to it's highest point and let the receiver handle the crossover. There are a lot of factors that allow you to hear the sub's location (localize). In no particular order some would be: location, volume and crossover point. The first thing you need to do is adjust the volume of the sub so that it is close to the mains. This is normally done with test tones and an SPL meter. Then you can adjust the crossover, if needed. If you are going to use the sub's control, once you have the level set, you can then dial in the crossover on the sub to blend (by ear basically) by having someone turn up the x-over until you can localize the sub and then back it off. This can be done with an SPL meter as well, by taking measurements of the in room response and adjusting the x-over to give you flat response, but that can get a bit involved.
