At my school's open day we had a speaker and were playing with frequencies, all the parents were amazed when their kids could hear things that they couldn't.
Same headphones here and I couldn't hear anything past 16k too.
My parents dont believe me when I can hear things they could not, some months back I could hear the TV but they could not, well weird.
Yep, many speaker will exceed xmax at 20hz unless the volume is really low. If anything this test is more a test of your soundsystem than your ears...Please remember, speakers distort the sound so what you hear is most likely not the true frequency.