Does the Rel line level split the signal through an internal crossover then speaker line goes filtered to the speaker?
No. REL recommends running the main speakers full range.
Solid State fully differential amps can NOT be used speaker level with REL subs you will damage the amplifier.
REL has instructions for connecting speaker level to fully differential amps (this link for a single subwoofer. There are instructions elsewhere for a stereo pair).
https://rel.net/blog/2015-07-23/how-to/how-to-connect-a-rel-to-a-balanced-amp/