A passive speaker level crossover with a corner frequency suitable for a subwoofer is impractical. You would need really big iron core inductors that would be difficult to source off the shelf. The crossover would be big and heavy and potentially limit power handling. Also, the results could be a little unpredictable due to interaction with the complex speaker impedance and amplifier output impedance.
For home theater, I'd set the speakers to small and use the receiver's crossover. For stereo music listening, you could try pure direct mode, which skips the crossover in the receiver. The front L&R speakers would run full range, and the crossover on the REL can be set to match the roll-off of the front L&R, just as REL intended. You could switch out of direct mode for movies, which will then filter the front L&R speakers so they are not over-driven, and the bass in those channels will get to the sub via the line input. I'm not the biggest fan of REL, but it's nice that they are able to mix the high level and low level inputs together.
HT: Dynaudio C2, Contour S CX, 2x BM14S, Aperion surrounds, Simaudio Titan, Marantz AV8801, Oppo 103, Linn Majik DS, and a Pioneer Kuro
Stereo: Dynaudio Focus 160, Simaudio W-5 LE or Luxman M-600A, Linn Akurate DSM, 2x Rythmik F12G
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