In my experience the REL's work best letting your main speakers run full range, with the REL filling in the bottom.
It is OK to leave some gap between the where the main speakers end on the low side, and where the REL takes over. You can then turn the volume higher on the subwoofer.
Many people setup the sub right up to the limit of the main speakers. This usually requires the volume on the sub to be set lower to reduce boom at the crossover/cutoff point, which negates some of the benefits of having the subwoofer.
It is OK to leave some gap between the where the main speakers end on the low side, and where the REL takes over. You can then turn the volume higher on the subwoofer.
Many people setup the sub right up to the limit of the main speakers. This usually requires the volume on the sub to be set lower to reduce boom at the crossover/cutoff point, which negates some of the benefits of having the subwoofer.