You have asked a really pertinent and important question, seldom asked and even less understood by even professional designers.
The place to start is understanding this chart.
This shows the frequency range of common musical instruments and speech.
It also shows the the fundamentals in black and the harmonics in yellow.
Now there is more power generated by the fundamental than the overtones. Most instruments radiate even harmonics. That is to say if the fundamental is 400 Hz the second harmonic will be 800 Hz. Generally the power output goes down as the order of harmonics increases.
Now if you look you will see a heavy clustering of fundamentals from 60 Hz to 600 Hz, but going out to the 2.5 to 3 Khz range, after that range we are largely dealing with harmonic content. The big exception is pipe organs that have powerful fundamentals throughout pretty much the whole audio range. This makes them ideal for burning out tweeters.
The big point is that most power is required in that 60/80 Hz range to the 600 to 1000 Hz range, but significant power also required to the 2.5 to 3 KHz range.
But it is worse for speakers then it looks. Most speakers have forward facing speakers. Now for every speaker like that there is a point where it transitions from being a forward (half space) monopole radiator to a omni directional (full space) omnipole radiator. This occurs at a frequency depending on the front baffle width. The narrower the front of the speaker the higher this transition frequency. This is known as the baffle step frequency as at that transition the forward radiation goes down 6 db. For most speakers this occurs between 400 and 600 Hz. So unless the range below this frequency is boosted the speaker sounds thin. So this requires more then doubling the power output below this transition. That frequency unfortunately corresponds to the area where fundamental frequencies of instruments are concentrated.
So you are correct in your concern about lack of power bandwidth in this frequency range. I personally believe that most speakers are deficient in the required power band response in this region, especially since small bass mids have become the rule.
There are very few small bass mids that do have sufficient power response in this band, but there are some.
In practice it means that more than one driver to cover this band is actually required.
If you really understand what I have just explained you will see why the notion that bookshelf speakers and a sub have you covered is nonsense.
One last issue is that properly compensating for the baffle step results in an impedance drop below the baffle step frequency. So to drive a properly designed speaker with correct baffle step compensation really requires amplification that is comfortable with 4 ohm loads. This is a downside of more channels being added to receivers, as there seems to be a universal backing away of late from specifying performance at 4 ohm.
