(We will presume here that the subwoofers have internal power amplifiers, as it makes the explanation simpler. The same would apply however if the power amplifiers were external, it's just a little more hooking up to do.)
Your question does not specify whether you are using your subs for monitoring in the studio, or for live sound. Let's assume for now that you are in the studio.
We do not recommend connecting the subwoofers directly to the mixing console in any way.
The reason for this is that every studio needs a monitoring system that above every other factor is consistent. It's nice to have a wide frequency range, nice to have low distortion, nice that it goes loud enough.
But all the other factors take second and progressively lesser places in comparison with consistency. If your monitoring is the same from day to day, you can learn to work around any imperfections. And since no monitor system is perfect, this will always be the case.
If your monitoring changes from day to day, then really you won't have a clue what you are listening to and your mixes will be dreadful.
So the only possible reason for connecting the subs to the mixing console would be so that you could make adjustments, and that is precisely what you should not be doing at the console in the studio.
All adjustments to the monitoring system should be done among the crossovers, amplifiers and loudspeakers - nowhere else. You should take as much time as you need to optimize your monitoring. And once you have decided on the best settings, leave it alone!
Setting up a subwoofer system is easier than it used to be.
In the 'olden days' the monitor output from the mixing console would connect to a crossover that would separate the mid and high frequencies, which would go to the power amplifiers for the main monitors, and the low frequencies, which go to the subs.
These days, the crossover is more likely to be built into the subwoofer. Take for an example the Wharfedale EVP-X18PB. This has a single 18 inch drive unit powered by a 400 watt amplifier.
In addition however it has connections for the left and right stereo signals from the monitor output of the mixing console. These lead internally to a crossover that separates the lows from the mids and highs.
The mids and highs go to two outputs, which you can then connect to the amps driving your main monitor loudspeakers.
The lows from the two channels are summed and are used to drive the sub. The sub has a level control so that you can blend it with the output of your main monitors. There is a phase switch too - to test this put the speakers close together, and use the setting where you hear the most bass.
Combining the two channels into one sub is a useable option. Low frequencies are not particularly directional. Of course it's better to use two subs if you can afford it.
The key to using subs successfully for monitoring is to match the output from the main monitors and the subs at the crossover frequency. This is difficult to do unless you have an acoustic level meter, but if you play the subs on their own and listen to the highest frequencies they produce, then lock these frequencies in your head and listen out for them when all of the monitors are playing. Balance the sub(s) so that this band of frequencies is at the same subjective level as all the other frequencies.
Although in theory in live sound it would be nice to think of the main speaker stacks as gigantic hi-fi speakers, in practice there are benefits to be gained from subjective optimization from venue to venue. For a fixed installation, studio practice as described above applies. For a traveling PA, then you can either do it the 'proper' way, or connect the subs to the console so that you can tweak the sound more easily.
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