My reasoning for high bandwidth is this: It takes about 7 sine waves to make a square wave using Fourier theory, I believe.
This if you can hear a 3 Khz tone easily, then 21 Khz would be necessary to reproduce a square wave at that frequency. That is attack time. e.g. Listening to a hammer hitting the strings on a piano.
We can obviously hear past 3 Khz and even at 10 KHz, that's a 70 Khz bandwidth requirement.
So, the 20-20 Khz frequency requirement is bogus in my opinion. My professional audio stuff goes from 0-100 KHz.