Okay, here's the effect of a 470 ohm in-line resistance on the signal fed to a pair of Sennheiser HD 535 headphones. They are headphones with neither the most variable, nor the most even of impedances across the audible frequency range. This graph ought to be more satisfactory. It's generated by RightMark Audio Analyser 6.4.5. It's pretty much the same as the white noise graph I showed above. As Restorer-John says above, it's wise to choose headphones with a flat impedance since they'll be immune to frequency balance variations caused by the voltage splitting differently at different frequencies. But impedance graphs for headphones aren't routinely published, so it would be preferable if electronics didn't allow their output voltage to be much affected by headphone impedance. That is, it would be best if they would keep their internal impedance low.