A while back @JohnM (creator of REW) mentioned an alternative way to present distortion. I believe he linked an AES paper. If someone can find that post, I'd appreciate it. At that time, I sort of discounted that approach because I felt normal distortion presentations were sufficient and already good enough - it was already kind of hard to get people to understand them anyway. Another kind of presentation would probably just cause confusion. Over time, I'm come to realize that distortion plots are still difficult to understand, so I've been playing around a bit. I don't know if I recall that AES paper correctly, but here is an approach I think we should consider. Normally, harmonic distortion (per order) is plotted at the fundamental with a different color. Second, third, fourth, etc. harmonics of a fundamental frequency, say 100Hz is plotted at the 100Hz point. It's up to the viewer to understand the gist of what second order distortion or third order distortion plots mean - that is crap at 200Hz and 300Hz that shouldn't be there when playing a 100Hz signal. This technique shifts the second order plot right to the second harmonic and the third order plot to the third harmonic. What this does is better illustrate the frequency of where the distortion actually manifests. The twist here is that the distortion signal is then taken relative to the amplitude (from the frequency response) at the frequency of where that distortion lies instead of the fundamental. For example, the amplitude of third order harmonic distortion of 100Hz is not measured relative to the 100Hz signal, but relative to the amplitude of the FR at 300Hz. With headphones, there are some good reasons for using this novel approach. For many normal distortion plots, we will see distortion spikes that appear an octave below FR spikes. These spikes really don't make much sense because the distortion probably isn't "real" because the distortion at those points is exacerbated by resonances inside the cup and/or cavity between the ear and the driver. By shifting the plot right (according to order) and also plotting against the amplitude at that point on the FR, we may get a more understandable or realistic presentation. If anything, distortion spikes, if they are indeed greater than what would be expected to be caused by internal resonances, should align at the points of these resonances from a visual presentation point of view.