Given that the crossover accepts RCA only, your best choice is XLR-to-RCA cable. This is the purist approach, which does not add any "processing" to the audio signal, be it an active converter or a passive transformer.
Be sure to terminate the unused phase of the XLR in the recommended impedance and not in a short, like most adapters do!
Be sure to terminate the unused phase of the XLR in the recommended impedance and not in a short, like most adapters do!