There are many types of sound sources that do not require subwoofer reinforcement since they lack low frequency content. For instance, most signals captured by microphones such as voices and instruments in general, except those that do have low frequency content, like double bass or kick drum. Moreover, noises such as stage rumble and handling noise leak into the microphones, as well as sound emanating from the main PA and the stage monitors. It is therefore beneficial to remove the low frequency content on such signals.
To filter out unwanted low frequencies, the built-in high-pass filter of a mixer's channel is often used (most serious live sound consoles would have this feature). It attenuates frequencies below the cutoff frequency, which is typically in the 75 -100 Hz range, depending on mixer model, although some have it sweepable. The filter slope is commonly 12 dB per octave.
The slope of this high pass filter may not be steep enough to filter out unwanted bass effectively. Or the cutoff frequency may not be low enough. There are also mixing engineers that wish to add a different amount of subwoofer level to each instrument. In those cases, the subwoofer way is fed through a post-fader auxiliary bus on the mixer. Thus, the channels that have the aux level at zero work in a similar way as if they had the channel high pass filter switched on, except for the fact that the filter characteristics are now those of the electronic cross over, which are typically 24 dB/octave, and a cutoff frequency which is that of the subwoofer way.
This way of mixing needs an additional cross over channel. These days digital processors have become cost effective and therefore popular, and they often feature a spare input that will be fed by the mixer's aux, so that a single processor can be used to accommodate aux fed subs. However, if we are using a more traditional cross over or controller unit, we will need an extra unit.
Driving the subwoofers from an auxiliary would normally mean a monophonic subwoofer channel, although there is no reason why two different auxiliaries could be not used to provide a stereo subwoofer channel, other than the fact that it may not provide much of a benefit and would eat up two many mixer auxiliaries and make the arrangement complicated.
With respect to assigning subwoofer levels on each channel auxiliary bus level, there exist two distinct schools of thought. The first, more technically correct and purist, aims to reflect the same frequency balance that would be achieved without driving the subwoofers separately. We must therefore calibrate the channels' aux levels as well as the main aux controls so that the subwoofer level is the same as if we had driver them in the conventional way.
The second school of thought would be more liberal, and would assign subwoofer level on each channel subjectively as required by each instrument.
The illustration below shows the configuration for a 3-way active system with subwoofers fed from an auxiliary mixer bus, compared to a similar system with subwoofers driven conventionally.
Conventionally fed subwoofers
Aux fed subwoofers