Actually the original question of this thread is incomplete as it begs for the counter question "... at what level?"
After all, without proper calibration, still being able to hear a certain frequency unfortunately doesn't mean that there might not already be significant hearing loss when one only turns up the gain high enough (besides possible occuring artifacts due to aliasing or other distortions).
For me, more interesting than sine tones though is to high pass-filter certain musical pieces and listen to what is still there at not too loud levels when taking the whole thing as reference.
Naturally, it is highly dependent on the source then, but enlightening nonetheless. My personal disappointment to not really hear any difference whatsoever when turning down the EQ around 20 kHz (and also 16 kHz actually) by 20 dB, I could halfway comfort by this article:
True audiophiles of course rail against any lossy codec and especially any which dares to touch anything only so slightly below 20 kHz, but the blind test results speak a different language as usual, again confirming that human ears aren't that great after all (at least in terms bandwidth and wear).