That seems like over-solving it. Sure, the LS50 performance tails off below 200Hz, but crossing up that high is overkill and just causes a different set of problems. Crossing as low as 80Hz already gives dramatically reduced excursion compared to the way it behaves unfiltered. Lots of people seem extremely happy with their LS50s totally unfiltered, where the midbass driver will be slamming in and out, so to suggest it needs to be crossed as high as 200Hz seems like hyperbole.
Anyway, another random thought - is a passive high-level HPF for the speakers an option? Seems like it could be the answer, but I can't see anything readily available off-the-shelf. I guess filter results depend on speaker impedance too much, so it's not a viable generic product?
If there's no theoretical problem, building your own passive HPF - with advice from someone in the DIY speaker area for component value+selection based on the measured LS50 Meta impedance curves - might be a fun little project. And potentially the cheapest option, even if you have to buy a soldering iron.
Well, it depends how steep the crossover frequency is. Crossing HP at 80 Hz 6 dB/oct with passive components, ( a simple 100-150 uF capacitor in series with the + terminal will do fine ) doesnt releave the bassdriver in ls50 meta from big excursions. But it can be better integrated with a subwoofer if doing so.
A dsp in series with the signal must be transparent enough to not destroy the qualitys of Kef ls50 meta. With a dsp you can do HP crossover at 100 Hz with 48 dB/oct, which is a big benefit if one wants to reduce cone-excursion.
But the signal must go trough an AD/DA extra, making it less transparent.
I dont think a cheap mini-dsp is good enough.
Making the kefs really good can surely be done by bypassing the passive crossover inside the loudspeaker, and making two terminals extra on the back of the loudspeaker, making the whole loudspeaker active with a dsp crossover and two power amps.
Then one can use a 48 dB/oct filter for HP at 100 Hz , and enjoy the benefit of actively drive the ls50 with a crossover at 2,1 kHz for tweeter and midrange. If the dsp crossover is good enough ( meaning it has to have digital inputs ) then the sound will be clearer than any passive options.
And such a speaker, optimized with dsp in the crossover between tweeter and midrange and with two subwoofers in stereo, would probably beat the Kef r7 soundwise.
The NHT xd system is made in this way. Measurements has been done by stereophile.
https://www.stereophile.com/content/nht-xd-active-loudspeaker-system-measurements