Etymotic MK5 IEMs

It really depends on the distortion response of the driver, which is usually separate to the frequency response. In the vast majority of cases this isn't an issue. Very loud low frequencies on the other hand tend to have a much easier time distorting other frequencies as they require the driver to move much further in order to achieve the same volume. Another thing to consider is that distortion presents itself as an increase in amplitude or creation of higher frequencies or harmonics of the original sound/fundamental. This means that if you have a loud sound at 18khz, the resulting distortion will most likely be above 20khz. In fact harmonic distortion occurs in a fairly repeatable graph. (This is ignoring other small things that occur near the fundamental as they are usually so much lower in volume that they are the last things to be heard.) Harmonic distortion tends to follow a pattern of double the fundamental frequency, with each next harmonic being half of the previous gap. (10hz fundamental, 20hz first harmonic, 25hz second harmonic etc etc. Note: this is simplified and actual harmonics aren't perfect integers like 1x 2x 2.5x 2.75x.) Point is that if you have a sound at 18khz, the next part of the harmonic distortion is going to occur at 36khz which is well above audible range.

Would it possibly distort the sound if 20kHz was pushed into a headphone not rated to go that high though? I know Ety products are supposed to be very detailed and I wouldn't want that to get messed up by pushing the limits of less-capable drivers. In testing I can usually hear up to 19.5-20kHz fairly reliably.