Subs on the floor dont function well above 100 Hz , and should have at least 24/dB oct lowpass filtering if only one is used . However, crossing a single subwoofer that high ( if its on the floor ) will bring a bad sound result.
If two subwoofers are connected very close to or below each main speaker and in stereo, or the two stereo coupled subwooferdrivers is at the same hight as each of the the main midbass driver, one can get away with a 6 dB lowpass crossover - this will then be a bass reinforcement speaker and can bring very good sound results.
If I had a pair of ls50 meta I would build two tower shaped subwoofers with the same hight as the metas, placed very near each ls50, in stereo.
The ls50 should be plugged to behave as a closed box. Because of that, I would use an inverted 12 dB/oct stereo lowpass crossover for the stereo subs. Probably around 70 Hz , near where the meta:s roll off as a closed box .
This way, no high pass crossover is needed for the metas.
An even better result can be had with a real separate stereo crossover with both HP and LP filtering .
One option is to use a passive line level crossover to roll off the closed ls50 with 6 dB /oct , then the subwoofers should be crossed with a noneinverted 18 dB/oct crossover .
Combined, it will be a 18/18 LP HP acoustical crossover .