For audio systems that incorporate limited range speakers, the crossover frequencies need to be selected.
The classic approach is to select crossover frequencies based on speaker specifications, but these may not reflect how the speakers perform in the room. It is better to measure the actual speaker performance. Use saved measurements, to compare speakers and select a crossover frequency.
The screenshot below shows a 2.1 desktop audio system consisting of main speakers and a subwoofer. The speakers were measured separately with crossovers disabled and no equalization. The subwoofer was measured first and saved (grey), then the main speakers were measured (green).
The published frequency range for the main speakers is 80-20000 Hz and the subwoofer is 35-165 Hz. Based on the specs, the overlap of these speakers is 80 - 165 Hz and the crossover frequency should be somewhere in that range.
On the magnitude plot, we can see the main speaker drops off quickly below ~120 Hz. For this system the crossover frequency should be somewhere in the 100-120 Hz range. The subwoofer is more than capable of filling in below 100 Hz.