My mixer has only two outputs: one for the loudspeakers and one for the monitors.
My active loudspeakers and active monitor speakers are already connected to these 2 outputs and work fine.
Now I also got a pair of active subwoofers and would like to connect it to the same mixer. How can I do?
Active subwoofers tend to split off their own signal and provide the rest, so they usually have not just an input, but also an output where the signal without bass appears. You don't want to mess with the output of your monitor speakers (as they are for, well, monitoring and the kind of muddy oomph you use subwoofers for does not make a lot of sense there), so you'd likely use your loudspeaker output as subwoofer input and connect your loudspeaker input to the bass-less output of the subwoofer.
What gear do you have? As other have mentioned, many subs come with a built-in crossover which splits the signal. One part goes from the crossover to the sub, and the other part is typically sent to a set of outputs so that you can link the subs to the other speakers.
If this isn't the case, have another look at your mixer. Chances are that you have a set of aux sends; typically labeled monitor or FX with a mono jack output. I've got a similar setup with monitors running on the console main and sub connected to the FX send. The FX-send is post-fader, so that any volume changes on the fader also changes the volume on the sub. Any aux send will work, but if the mixer doesn't have any post-fader auxes you might have to control the sub volume using the aux send or aux master knob.
