When the LSW was first designed in 1984 the intent was to use it with Quads at a 175 Hz crossover. Since a 12" driver develops roughness at a much higher frequency than a 15" I included it, along with the 15" active and the 15" passive to move a lot of air. The combination worked well.
No 15" by itself provides a smooth enough transition for my taste regardless of crossover point, therefore I augment the big driver with a smaller one.
