As painttoad points out, the mains need to be small for the sub to be active when in stereo mode (Pro Logic will send information to the center which could then be crossed over and sent to the sub). You also need to make sure the material that you are listening to has content below your crossover point - a lot of music doesn't. (My DTD can give you a CD or DVD with a low frequency tone that can help test this.)

The only way to "turn off" the mains while the surrounds are active is to disconnect the speakers.