Don’t discount the driver differences.
The blades crossover from the woofers at 300hz based on measurements. Kef claims 350hz.
the sub will be fine at 80hz and down but there will most likely a sound power issue between the 80hz and 300hz crossovers if you ran the SL50 with subs. The big heavy drivers of subs don’t play really nice above 100hz. I am sure there are a few out there though.
At low level there might not be too much difference if your not critical but push the sound pressure and integration issues and distortion/compression will surely show it’s head
Personally I don’t care for subs with small speakers and I think they blend a lot better for critical music listening with larger speakers. Think of it as turning your large 3 way into a very large 4 way.