The benefit of having a subwoofer
Subwoofers have not been invented without a good reason. The main reasons why subwoofers exist are related to the limitations of the monitors to either deliver the lowest frequencies in the first place or then to enable reproduction of the lowest frequencies with good quality. The first reason is related to the monitoring loudspeaker capacity and the second reason is related to the listening room acoustics and how the monitors interact with the listening room at low frequencies. Subwoofers can help with both.
Improve and correct the low frequency coverage
This is the most obvious reason for having one or more subwoofers in a room. Your monitors do not have the bass extension to actually output the lowest frequencies. Using the monitors alone, there will be an inevitable roll-off and your perception of the lowest frequency range will not be correct. Subwoofers can help here and restore the level of the lowest octaves. This helps your mixes to sound right.
Reach higher SPL
A not-so-obvious reason for using subwoofers to support monitors is to enable the monitors play cleaner audio and reach higher SPL levels. Reproducing the lowest frequencies is a very demanding task for a monitor. The woofer cone must move a lot at the lowest frequency output and that is also where the highest power in the monitor is usually needed. When you bring in a subwoofer, this demand reduces to a much more manageable level. When the woofer moves a bit less, monitor linearity improves. At the same time, more amplifier power is available to reproduce higher frequencies, and this translates to cleaner, more dynamic audio output in the monitor. So, subwoofers can really support monitors to play better cleaner audio.
Flatter system frequency response
Then we come to the room interaction. Monitors are frequently not placed such that they could reproduce a flat frequency response at low frequencies. Room acoustics dominate at low frequencies and correct location for the monitor may not be ideal for low frequency output. When subwoofers are used, it becomes possible to move the lowest frequency output to a physical location in the room where low frequencies can be output with high efficiency and so that the frequency response remains flat at the listening location. Because of this Genelec asks you to typically place the subwoofer very close to walls on the floor. This avoids destructive reflections at low frequencies and effective acoustic coupling to the room acoustics, to output audio efficiently. With subwoofers available for low frequency output your total system frequency response can be much more neutral.
Freedom for monitor layout placement
Particularly with conventional monitor designs it is necessary to place the monitors at equal physical distance from the listener to achieve the correct timing for audio. This is needed to ensure that the stereo images are located correctly. Then, having one or more subwoofers in the system can be a real help. You can achieve a flatter system frequency response and there is much more freedom in placing the monitors when subwoofers are used for low frequency output.
More subwoofers can increase maximum SPL
One subwoofer, shared across all monitors using Bass Management can be a great help. There are several reasons why more than one subwoofer could be even better. The most obvious benefit of using multiple subwoofers is simply more output capacity at the lowest frequencies. This may be needed to balance the output capacity with the monitor system. If the subwoofer is not able to reach the same peak output levels as the monitor system, there will be a limitation to the dynamic range of the system, and this may not be immediately obvious. Without careful understanding of the total system output capacity, it is possible to monitor at the level where the system is not delivering correct understanding of the low frequency level and balance with higher frequencies.
How does the level increase with multiple subwoofers?
If two subwoofers are place close to each other to play the same audio, the level gain in the room is about 6 dB. To gain another 6 dB you will need to double the number of subwoofers.
More subwoofers can improve frequency response flatness
One subwoofer can be placed in the best position in the room to achieve flat frequency response but typically there can be frequency ranges where the level is less than optimal. With the second and third subwoofer it is possible to select the subwoofer locations so that subwoofers complement each, summing up to an even more flat frequency response. This will provide a system with a very solid and capable bottom end. Using more than one subwoofer can be a game changer in terms of response quality.
A subwoofer can extend single monitors
Naturally, the best situation would be that your monitors have the range extending to the lowest frequencies. This is possible when a subwoofer is sharing the workload for an individual monitor. Then, it is possible to place the subwoofer so that the direction of audio remains the same as with the monitor, yet the subwoofer can be placed close to the wall and on the floor, to enable powerful low frequency output. With the GLM calibration, such a system can integrate into a single, very capable full range monitor, giving you the access to excellent stereo imaging extending down to the lowest frequencies. Using one subwoofer for each monitor is an approach for those looking for the highest fidelity and precision.
More subwoofers can improve envelopment
Envelopment means that you feel there is a real sound field and that you are inside it. One subwoofer is interacting with the room predominantly in the direction of its placement. With more than one subwoofer, it is possible to create low frequency reproduction that provides better and more involving envelopment and access to the low frequency details recorded in your mix.