Over the years I've read on multiple occasions to take the recommended or stated lowest frequency that your speaker in question can deliver and double it for the crossover. {Eg. 40hz. to 80 or 60hz. to 120}.
If you check some of the Subwoofer specialty companies or popular models. This is their common recommendation. When using line level or RCA jack.

Although if you check the REL sight and lots of stereo aficionados they actually recommend the high level input to mate the subs to speakers. For matching signals"better"