A full range speaker?

This may be useful for some - here's a Sound on Sound page with a link to a frequency chart in the top right corner. Fundamental notes are within the range of 16Hz to 9kHz, with harmonics going much further up in frequency. If you don't listen to a lot of pipe organ music then a lower bound of 30Hz will include all the fundamentals e.g. if you mostly listen to rock music low B on a 5/6 string bass is 30Hz and you'll hear everything a 22" bass drum has to offer from around 55Hz up.
Providing you have a loudspeaker that will play low enough the room will dominate the frequency response below the low 100s of Hertz. As loudspeaker placement is key to getting the best low frequency response it can be beneficial to allow one or more sub woofers to take on this duty. In a large, acoustically controlled space this is not so much of a problem and it matters less if the low frequencies are coming from the same cabinet as the bass, mid and high.
The high frequency response is bounded by our ability to hear and 20kHz is well outside of this range assuming most of us are beyond our teenage years. There may well be recorded content above this arbitrary limit but I won't be able to hear it and so I don't miss it if it's not reproduced.
So for me 30Hz-20kHz is fine and I'm not sure I'd notice too much if it was a smaller range than that. I build equipment (not loudspeakers) to exceed 20Hz to 20kHz because that is the defacto standard.

djones, a rising response from 100 Hz down to 20 Hz with 20 Hz up 6 dB does not produce fat bass. Fat bass is produced by a peak in the 80 to 160 Hz region. Pushing the response up at 20 Hz gives you that sense of a live performance at less than ear shattering levels. Push it too high and the bass becomes disjointed, you know you are listening subwoofers. You should never know you are listening to subwoofers. 

@motzartfan

It’s fairly simple in the end, the elusive balanced tone.

Driver relationship and harmonic fidelity are more dependent on surface area than ability to reproduce a particular tone at a geometric relationship to surface area.
Your sub surface area and your woofers relative relationship are more important than your subs ability to reproduce it’s lowest coherent frequency. 

In order to couple the two you should consider the Golden Ratio math equivalency relationship to produce natural lower extension to the main speakers.

Buying a bigger sub, surface area wise, with a lower power amp, and having to throttle it back is preferable to a smaller driver size with a bigger amp; all punch and no depth.

All drivers have unbelievable turbulence in the cone area, and smaller drivers magnify that by needing a greater X-max to reproduce a particular tone. Ideally, a 10” woofer should have a 20.1” sub to mathematically couple, sans room effects.

I’m settling for 18” short throw paper cone, extremely light and ridged, right now in my studio. And at about ⅓ power it feels perfect.