I took a similar test with my HD-595 headpones via my Beresford TC-7520 and found I was ok up to 16,500hz (-3db) in right ear and 16,500hz (-6db) in the left.
Up to 12,500hz I was spot on in both with no extra volume needed.
Beyond 16,500hz I could not really hear much. (I think the next band was 18khz)
At the other end of the audible spectrum I was fine right down as far as the test went (or as far down as my headphones went more likely) at around 25hz.
However, I am in my late 40's so I am very pleased I can hear 16,500hz at all.