Adding additional subwoofage can actually make the response worse if done improperly.
If you are happy with the sound of your system there is little reason to change it, but here is a pseudo-technical explanation of why you might want to add one or more subwoofers even if the one you have is already loud enough. I did it.
There are about a billion threads on multiple forums explaining the details but I summarized the high points for you here. You can do searches on any of the terms you have questions about and find abundant additional info.
Harman has a good white paper
on using multiple subwoofers
and room mode calculator.
Adding a second identical subwoofer of equal power adds 3dB of power gain.
You can hear the difference of 3dB easily if your receiver allows you to select relative volume setting and displays dB units on the volume control. 0.5dB is barely audible change to most people; 3dB is audible and readily detected but not overwhelming. 10dB is roughly doubling/halving of perceived loudness.
There is something called 'mutual coupling' that happens when the distance between the subwoofers is equal to or less than a quarter wavelength. Mutual coupling between physically separate subwoofers usually only applies outdoors or in a large venue where room boundaries are physically distant from each other. There is an additional 3dB of power gain for 6dB total gain. The transition to mutual coupling is gradual rather than abrupt.
To find the frequency that your dual subwoofers will get the full benefit of this mutual coupling gain, enter four times the distance between them into the wavelength-to-frequency calculator at the link above. For 8' separation the mutual coupling is fully established at ~35Hz and below.
The mutual coupling frequency is actually the frequency where physically separated drivers reinforce each other regardless of difference in distance to the listener because phase differences are small enough to prevent cancellation e.g. combing and lobing.
Above that frequency where mutual coupling ends, there is frequency-dependent and direction-dependent destructive cancellation (combing and lobing) from phase difference between the drivers if they are not equidistant to the listener.
Manufacturers often take advantage of mutual coupling between closely spaced drivers within a single speaker cabinet to improve efficiency over a broader range of frequencies than is typically possible with separate cabinets.
Mutual coupling between physically separate subwoofers is largely negated in a typical residential dwelling by reflections off the room boundaries that alter the bass response radically with phase-shifted delayed reflections and room resonances, even when subwoofers are placed very close to each other or stacked.
Subwoofers can have boundary gain below a certain frequency (similar to mutual coupling) from being acoustically close to a boundary, they can have boundary interference above a certain frequency (cancellation with phase-shifted reflected energy) from being acoustically distant from a boundary, and they can have either gain or cancellation from stimulating any acoustically proximal resonant room modes
that involve reflections from multiple boundaries.
Room modes create standing waves
(physically static interaction between waves travelling in opposite directions) at anything between gain or cancellation depending on the listening position within the path of the room mode that is stimulated. The axial modes are resonances due to perpendicular reflections between parallel walls; the tangential modes reflect off four surfaces and oblique modes reflect off six surfaces. You can see spatial illustration of room modes here
. Click on the buttons in the show: column to see the spatial illustration for each mode.
Multiple modes exist at all locations within a room but they are only stimulated when driven at a node and not stimulated when driven at an antinode.
I hope I got that node-antinode thing correct; I sometimes confuse the two.
The modes can get very complicated if the room is not rectangular but unless the deviations are radical i.e. nonparallel walls, major discontinuities, or curved surfaces my limited experience has been that the modes are still fairly predictable. For average small listening room the transition to modal behavior happens at ~300Hz and maybe a little lower say as low as ~200Hz for very large living room or dedicated home theater, and in that case mutual coupling between subwoofers never really happens at all in such enclosed space because the modal behavior dominates the response throughout their operating range.
Note that all of these considerations apply to the listening position as well as the subwoofer placement. The transfer function between speaker and listener is bidirectional; that is the principle behind using the subwoofer crawl
to find an optimized subwoofer placement.
For systems with smaller speakers with higher crossover frequency, the subwoofer placement might be more constrained due to the localizable nature of upper bass (above 80Hz). It can be distracting to hear the bass from vocals in your center speaker redirected to a subwoofer next to your sofa but at least for the main listening position left-right symmetry can largely ameliorate this effect.
Boundary gain at very low frequencies (below where modal behavior ends) from reflections off all boundaries simultaneously is called cabin gain. For an approximately sealed room, cabin gain translates into a direct pressurization of the space. The gain is approximately 20dB (4x louder) per decade (frequency/10) and is very effective way of getting big bass from small speakers in a car particularly with all doors and windows closed. Cabin gain occurs at much lower frequencies in a typical residential listening room and is helpful for infrasonic tactile/visceral sensation in high performance systems. Cabin gain is unlikely to have effect at audio frequencies in a large venue and does not exist at all outdoors.
All reflections except boundary gain have these effects at multiple discrete frequencies per reflection, i.e. 1x, 2x, 3x, 4x etc. so the response can get very complicated to predict based on analysis
alone. Simulation may be required to predict the response and the accuracy of the modelling is limited by how well the room and its objects can be described mathematically. Subwoofer placement and integration is therefore usually done by ear or with measuring equipment instead because most listening rooms are neither rectangular nor devoid of objects.
In the lower bass where room modes are sparse the deviations in response can be drastic.
Uneven bass response is largely due to too few modes in a smaller room, not too many. As the room size increases, the modal density at lower frequencies also increases, filling in the non-resonant gaps between resonances and eliminating modal phase cancellation nulls. In the case of larger performance venue it is probably more advantageous to group subwoofers for mutual coupling at subwoofer frequencies rather than distribute them to directly stimulate specific room modes.
A good graphical representation of the frequency distribution of modes in a typical home listening room can be found here
(actually those modes are much more evenly distributed than those in my own room).
There is one more factor to consider and that is whether any of the boundaries resonate significantly. In my listening room it seems the floor resonates and adds a strong tactile sensation because it makes the whole sofa shake. This is considered desirable and leads many people to build home theater upstairs rather than in the basement on a concrete slab. Resonating room boundaries also alter the predicted response some.
At various frequencies the output of a subwoofer in a room can have anywhere from additional ~+30dB to -infinity dB of gain due to reflections and resonances. The actual output at the listening position depends on the shape of the room, any reflecting or sound-absorbing contents in the room that have an effect at bass frequencies (such as large tables, bass traps and sofas), and the placement of the subwoofer and listening position within the room.
When the variations are large the frequency response sounds colored. The broader the range of frequencies that are affected by gain or attenuation and the more drastic the variation between frequencies, the more perceptible the coloration is.
The major advantage of multiple subwoofers is to stimulate as many of the differing room modes as possible to produce more even bass response across a larger listening area than is possible with a single subwoofer. Additional output is seldom a factor but it can help especially with 4 or more subwoofers in a large room or when substantial output at very low frequencies is desired.
Another option is to stimulate as few of the lower frequency room modes as possible to avoid creating any strong resonant peaks in the response. In that case, total output can be significantly lower so more power or more subwoofers may be required to get acceptable levels.
A combination of resonant and non-resonant placement may also be used.
Note that at higher bass frequencies so many room modes exist and are evenly distributed at closely spaced frequencies that there are few to no issues with frequency response due to resonances.
It is possible to have better bass with non-equal phase and gain and frequency response between multiple subwoofers of differing design; in fact the phase gain and frequency response even between identical multiple subwoofers will nearly always be at least slightly different and usually drastically different due to placement of the subwoofers and listener across all the seating, and that is sort of the whole point of using more than one sub in a 'small' room (less than full sized commercial theater) that exhibits strong modal response.
Also, deliberately changing the phase gain and frequency response of multiple subwoofers is a commonly used advanced technique to smooth out the bass. Since I first posted this comment, I have added a third subwoofer (Hsu VTF-1 MK2) and am in the process of tuning it narrow band to fill in a 30Hz gap in my room where there are no room modes to stimulate.
So it is not necessary to have identical subwoofers but it may make integrating (matching them up with each other) easier in some cases, particularly when the room is symmetrical and the placement is also symmetrical. Most people recommend identical subwoofers to simplify the integration.