Audio video receivers (AVRs) have "bass management" which can be configured to route everything except the bass to the main speakers and re-route the bass to an active subwoofer. With movies the "point one" LFE channel (Low Frequency Effects)
always goes to the subwoofer. (You loose the LFE in the stereo downmix). Since it's built-into every AVR, that's how most people handle it.
For line level signals you can use an
active crossover. These are typically used in pro PA systems. Normally, the full-range signal is fed-in and split into 2 or 3 bands and all of the bands can be adjusted separately and each frequency-band goes to a separate amplifier (or active speaker).
The
miniDSP can also be programmed as a crossover. This is getting popular.
Some high-power pro amplifiers have DSP filtering so you can program the amplifier for one frequency band. i.e. Two amplifiers for a 2-way system or 3 amplifiers for a 3-way system, etc.