Perplexed on how single driver speakers can cover such a large Hz range

It is quite simple however it is also difficult to implement. You also need to consider what is "Full Range". Very very few speakers reach into sub bass territory yet most people consider most box speakers full range. Most speakers reach into the 40-50hz flat region and that can be accomplished via rear horn with a good AER, Voxativ, Lowther or even a Lii Audio but I am only a fan of Lowther if rebuilt by one of the gurus or the Field coil version. 

You need 2 things to effectively pull it off and it leads to a driver that needs to be implemented in a very specific manner. I am a member of the "Lowther Modification" group on Fb where several of us , myself included make our own high eff drivers completely from scratch so I know these driver types intimately. 

I also own AER BD3 drivers in addition to the ones I build and 5 other FR drivers and there is a definite trend in all of them. 

In order to reach the very high frequencies in a large driver you need a VERY strong motor and a VERY light cone. The cones in these have employed long fibers, stiffening ribs and rolled edges for quite some time and those features are just now becoming standard in ScanSpeak, Eton and others as they play catchup.

The strong motor, light cone formula leads to a rising frequency response with a VERY low eletrical Q  that is best used in a back or front loaded horn. The rear horn will give the best low frequency response from a driver that barely needs to move to make bass. As the motor strength decreases so does the HF extension.

If they are not in a rear horn then bass response will suffer along with efficiency. 

The really good drivers are not cheap and should not be confused with the cheap kit drivers. The real good drivers start at $10,000 a pair and that is just the driver.

While back loaded horns and "full range" speakers of this type are not as popular in the US there is a new US manufacturer, Songer Audio, that makes an outstanding field coil "full range" speaker w open baffle and integrated sub that I heard 3 weeks ago in the LA show that is incredible. 

I think you can get full-range drivers to play some good bass, in-room.  But the cost would be cabinet size.  You can take a larger 10"-15" full-range driver, but with their reduced xmax, they need a big enclosure to reach into the lower frequencies.  Something like 8, 12 or more cubic feet in size to get the appropriate bass output, which most people can't or don't want to sacrifice.

marshinski15

You have some great help here from the replies.  I will will give you my take.

First up, in the normal run of things you would like to have a full range of frequency response.  But (1) the midrange is where most of the music is; (2) The avoidance of crossovers is a benefit - but this is (a) mitigated by quality (whatever that is) designs of the crossover and (2) systems (eg  ATC) where all three drivers in a three way system have similar cone designs.  (3) And this is my main point: it depends on your music.  Say your real interest is in string quartets: then the benefits of a single driver come to the fore.  Disclosure: I have not heard a single driver system. 

They play into break-up modes, that’s how. 

If you get a chance, give a listen to the Manger speakers. The Manger MST covers a large part of the frequency range, assisted by one or two 8” woofers.  Different technology altogether and extremely well executed.  Active or passive models in modest but elegant enclosures.  Nothing else like them on the market in many respects.