A mistake? a lot of users make at first is settling the REL frequency cutoff to high, and then to compensate for some bass bloat, they turn the volume down. As you lower the cutoff point, you can in-turn increase the volume. This will bring out the deep bass entension.
The other post are also good. For some tastes, rooms or uses the user may want to supplement the bass the speakers put out by choosing a higher setting that overlaps the speakers lowest bass. I discussed this a while back with Mike Smith at Ambrosia Audio and his response was that in their experience, if the listener wants to supplement the main speaker's bass, it is better not to put the Sub in the corner as is usually the case. (Still against the wall, just not in the corner). Moving the sub toward the mid-wall will mitigate any boom or bloat from the overlap.