Peter - Studies have shown that even though listeners cannot hear single frequency tones above a certain point, they usually can detect a difference when those same high frequencies are removed from a musical signal containing those frequencies as higher harmonics. I think that's what you're referring to. Also I hope that you made a typo stating that you "cannot hear 15hz and above" and that you meant above 15kHz, otherwise none of this really matters!
Good luck with your tweeter experiment and please share your results with us.