The largest subwoofer in the world is limited by the speakers with which it is associated. This may not hold true for all systems but, in genral, it makes sense to me. Moreover, the limits of average system loudness lies with the dialogue. Turn up the volume too much and the dialogue takes on the qualities a deaf old granny listening to her TV! No offence to grannies. Just making a simple analogue. Having set our steady state upper limit on dialogue SPLs one cannot just turn up the subwoofer alone. Or it all becomes too bottom heavy. The dialogue would be simply drowned out by the bass. Running the subwoofer hot to get a bit more excitement into the LFE is quite commonplace. The human ear actually seems to prefer a rising "housecurve" into the deep bass. This is probably tied in with the equal loudness curve. The human ear is insensitive to low frequencies. So the lower the frequency the higher the level needed to make it sound equally loud as higher frequencies.
You can't turn up the mains and surrounds either or the dialogue is lost in a sea of exaggerated environmental sounds. Though I often surprise myself how loudly my system is playing if I leave the room during a film then return armed with coffee and ice cream. I can still remember this being the case with cinemas. Though it must be decades since I last entered one. So a degree of over exaggeration in sound levels is acceptable and even expected. If only to add excitement and to drown ot the rustle of snack bags. This exaggwration of loudness is rather odd when one thinks about it. Because ordinary speech is not usually very loud in real life unless the person is "talking too loudly". Speech is only usually around 60-65dB(C) measured indoors where there is no distracting background noise, traffic, music or kids screaming. So whacking the master volume control up to give a constant 80dB(C) on ordinary dialogue is a bit of a distortion on everyday reality.
The answer to our sound level problem is wide dynamic range. It is the contrast between the soft parts and the loudness of noisy parts which makes the film exciting without singing the hair on (and in) your ears. The difference between peak loudness and quiet moments should be possible at all frequencies without audible distortion. It's no use having an IB to bellow out the bottom end if your speakers aren't up to their respnsibility for the higher frequencies. Otherwise the bass becomes too dominant on peaks of loudness too. Which may be a good thing if it protects the ears for suffering damaging peaks at dangerous SPLs at higher frequencies. Low distortion means fewer false harmonics are produced. So speakers capable of large dynamic peaks should actually be safer than those which struggle when pushed too hard.
Simply becauase they produce less distortion.
The response from those who value low distortion ansd wide dynamic range seems to parallel the IB. More drive units are brought in to allow them to coast along even when they are playing much louder than "normal" domestic stereo speakers. Usually they also have the advantage of being rolled off with active crossovers. Since they no longer need to produce high levels of low frequencies they are easier to drive which means the ampifier has an easier time too. Many of these multi-driver speakers use open baffle arrangements to avoid colouration.
