REL subwoofers are easy to set up and blend with your system. Run your main speakers full range and blend the RELs into your speakers. I would not be concerned much with specs because the room and overall system plays a big part in the balance. This really needs to be done by ear. Read your REL owners manual.
Subwoofer Crossover Question
What would be the starting point for Subwoofer integration with a pair of speakers that has the Frequency response below. Any layman explanation for what the -6db at 23Hz signifies and how this might relate to subwoofer crossover into a two channel music system, (two Rel Strata III's) will also be appreciated.
Frequency Response 28Hz to 25kHz ±2dB, -6dB at 23Hz
Frequency Response 28Hz to 25kHz ±2dB, -6dB at 23Hz