Poor crossover setup creates an audible gap (frequencies neither speaker nor sub properly cover) or overlap (both produce the same frequencies, causing peaks). The goal: the transition between subwoofer and main speakers should be completely inaudible — bass should sound like it comes from the speakers, not a separate box.
Optimization steps: 1) Set crossover frequency (typically 80 Hz for most systems), 2) Adjust subwoofer level to match speakers, 3) Adjust phase/polarity so sub and speakers reinforce rather than cancel at the crossover point, 4) Use acoustic measurement to verify smooth response through the crossover region.