Maybe someone has a sub with a 50hz cross over and the speakers go down below 50hz. If the speakers go to 40hz, for example, then there will be a 10Hz gap that both the sub and speakers respond.
I suppose this creates a "hump" in the music where there is too much midbase. The flow isnt natural and I guess it takes the focus off the rest of the music.

I guess thats 1 reason why you would want to turn off your subwoofer.