Lampy, the basic idea behind bass management is to allow the sub to handle the very low frequencies which it does best(at least if it's good enough)and to relieve the mains and other speakers set "small" of part of the bass burden so that they play the higher frequencies more cleanly. This also relieves the receiver driving the speakers from having to amplify the demanding very low frequencies. So, the point is to get the best overall sound, not to be concerned about wasting part of the frequency range of the mains.
