Originally Posted by zzffnn
^ Thanks. I would be highly interested to know why, if you are capable of providing such reasons.
It's actually not as easy to explain as one would hope, but I'll try.
Distortion is most easily understood by considering the case of a pure tone, a sine wave. A perfect sine wave consists of only one frequency, the fundamental, the pitch we hear. If you graphed the spectrum of a perfect sine wave there would be one frequency on the graph, and no others.
We now pass that sine wave through a device that creates distortion. Distortion is created by several types of non-linear behavior, meaning, the shape of a pure sine wave is altered somehow. When that happens, the sine wave no longer consists of just one frequency because harmonics are generated by the distortion of the wave shape. There are different mechanisms that cause distortion, and different kinds of distortion results. We've been talking about even-order harmonic distortion vs odd-order. Even order distortion would be the fundamental of the sine wave, say 1KHz as an example, plus the second harmonic at 2KHz, plus the 4th harmonic at 4KHz, plus the 6th harmonic at 6KHz, and so on. The specific amount of each harmonic is highly variable and depends on the mechanism that caused the distortion. But the key is, all distortion by-products are frequencies at even harmonics.
A different mechanism might cause odd-order harmonics to appear. This kind of mechanism is again very different from the even-order one, and produces harmonics at the 3rd, 5th, 7th, 9th, and so on.
So, in answer to your question (which was NOT stupid at all!) "since even order distortions are less audible than odd order ones, is there a way to covert odd order distortions of headphones to even order ones?", is that even order harmonic distortion is caused by a different mechanism than odd. To change the kind of distortion in a headphone we'd have to change whatever caused it in the first place, and that would be a new design.
However, if you had very accurate knowledge of exactly how the distortion came to be, and exactly what mechanism caused it, it is sometimes possible to create the inverse mechanism, to create "anti-distortion" (the correct term is "pre-distortion") and compensate in advance for the distortion that is about to occur. This won't work in all cases because some distortion mechanisms are so brutal and abrupt there no way to pre-distort for them. But if an amplifier has a lot of gentle odd-order distortion, it may be possible to feed it from a circuit that pre-distorts the signal in an opposite way so that the end result has less distortion in it. It's not a process you can just add to an amp, it has to happen in the design itself, though.
Your second question was "would THD+N be more audible on headphones or IEMs, assuming they both had the same inherent THD." The audibility of distortion during music is a very complex subject. Some music will mask distortion quite well, other music will expose it. But if your headphones and IEMs have similar frequency response characteristics, and similar THD specs, then we have to ask what would cause distortion to be more audible on one or the other?
Distortion, recall, is additional harmonic frequencies added to the original signal by some form of nonlinearity in the system. We hear distortion because we hear those new harmonics. We hear their frequency and level. We already know we can't change the frequency of the distortion harmonics, and if the frequency response of a headphone or IEM doesn't change their level, then the distortion wouldn't sound any different on either one. But if you have some headphones that have radically different frequency response, and they have the unfortunate ability to boost response around 3KHz, and you have a recording of a solo flute playing notes around 1KHz, and there's no other music content to mask the distortion harmonics, then you might hear the distortion more. But that's a whole lotta "if"s and "ands". Similar sounding headphones won't make distortion any more or less audible.