Some speakers (usually very expensive) really don't need subs or a sub makes very marginal difference. Then there's the question of power. Bass frequencies require lots of power while higher frequencies do not. By adding a sub with active cross-over and its own amp, you free the other amp to reproduce just the less power demanding higher frequencies. Two benefits: more head room less clipping and usually more phase coherence due to the coil-less electronic cross-over. Bottom line--much clearer sound with the subsonic signature alluded to above. If you need bass, this definitely the way to go.
