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