[Schalldampfer]

So... humans can't really hear above 20,000Hz, correct? Then is the headphone frequency response above that cutoff any useful?

 

[arterius2]

nope, anything above your hearing range will not have any effect.

EDIT: however, some people will "claim" that they can hear above normal audible frequency range. various auditory-acute animals can also hear beyond human's audible frequency range.

 

[Kosmic]

Hearing range degrades with age

 

[Audio-Fi]

All of us that works in the studio can hear 20Hz-20Khz. Some only can hear 40Hz-16Khz which are older people but not all of them.

 

[HFat]

How high (or low) you hear depends on the volume, right?

It don't think it's such a good idea to play really loud tones just to see if you can hear them... it seems to me it would make more sense to look at how high you can hear at normal listening volumes.
I'm twice as old as some members and I can't hear anywhere as high as 20K at normal volumes... probably no more than 14K and maybe as low as 10K actually. Even that's a stretch because recordings have normally very little content there.

Here's an idea: cut the frequencies above an arbitrary point (smoothly, without creating artifacts) and try to see if you can ABX a music sample with and without the lowpass. You might be surprised how low you can go without noticing it.