Bandwidth question?

Let's start with audible frequency bandwidth = 20Hz...20KHz.
Than take any full range speaker that would be less-likely designed to cover the whole audible bandwidth.
Than take onto consideration that vast majority of people wouldn't hear anything bellow 28Hz or above 16kHz.
Than let's see where 100 or 300KHz???

A square waves are usually considered as samples in digital carying or encoding frequencies that are absolutely unaudible just like DC. A sound is a continuous signal in general. As far as "rounding of the edges..." looks like heresy to me so don't listen and don't translate unlistenable.

Many amplifiers can go way beyond audible bandwidth, but last would less-likely dictate performance difference.

Some people hear more air with vinyl than CDs. CDs usually cut off at 20kHz due to filters, for the noise we may hear. Vinyl is capable of going to over 30kHz, if the cartridge and system allows it. I seem to hear more air with vinyl too. If I was given a hearing test, I'd probably not hear above 16kHz at the extreme. Maybe it is harder to identify a continuous steady test tone, than a variety of them, warbled tone, or music. I do hear some metal dome tweeters that resonate at 22kHz. This causes extreme an irritable fatigue that leaves me no choice of either shutting it off, or leaving the room quickly. So, I think we are aware of sounds up to this 22kHz (at least in my case) sound that may be there. When it comes to amps, I don't seem to hear any more air with an amp that may be down 3 decibels at 20kHz, in comparison to one that goes to over 100kHz.I have several audiophile friend that don't hear the upper end air from an amp that has a high bandwidth like that. Two are sensitive to that 22kHz noise like myself. Ultrasonic alarms also irritate us.

I don't seem to hear any more air with an amp that may be down 3 decibels at 20kHz

Let me clarify. I mean may be down 3 decibels at over 20kHz. In other words down 3 db at 22kHz for example.