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

Consider headphones. If the required output is small you can reproduce any frequency

They can’t , I have owned a bunch of these full range drivers, which they are not, and are not linear they may get high frequencies to 16 khz  if a electric field coil driver much much better ,but still need a powered sub to be a full range speaker. And with single drivers non field coil they cannot play with higher SPL levels at all they become peaky in the upper midrange ,included several mentioned which I owned,but will not disparage them ,and start distorting at around 90 db they can sound pleasant but still no bass to 40hz 

a big Electrostatic speaker like Sound Labs is the-only exception stretching the term single driver ,and needs lots of Power to play loud and they are Big speakers but sound great.
 

Then you have transmission line cabinets with single drivers that can go low. Sibelius is one such speaker. But yeah, it isn't for playing crazy loud. All speaker designs are compromises.  I can't remember but Zu or Tekton have a two-driver speaker where the crossover is a very high 12KHz to the tweeter. So the woofer/mid is really doing about 95% of the work and most people on the high side of 60 probably can't even hear the tweeter much anyway. 

An interesting oldie from the 70s was the Rectilinear X(ten) 3 way but the mid range ran from 200 Hz to 8 kHz with a 2 " cone tweeter above that and a 10" woofer in a closed box below so an almost full range speaker. And interestingly the crossover was a series one not a conventional parallel design.

But I agree with lots of us that full range has too many compromises in cut off lows and highs and inability to play loud. But I do love the top to bottom integration. You sense there is no crossover.

Just a closing thought. Perhaps the best full range speakers are a few one way planars and electrostatics. The most wonderful being perhaps the Quad 63 which even could image. 

The old constraints described above for crossoverless single-driver speakers have  been nearly, if not totally, eliminated:

https://www.cubeaudio.eu/reviews