The reason is very simple.
One point Amir makes is valid. The darn drivers clip and they clip hard above a certain point. You need to boost subbass a lot for this to happen.
The resonance point of these drivers is really low, much lower than most others.
Suppose we listen to music and not a sine at 20 Hz. Then bass notes and kick drums are about the lowest frequencies around. Let's forget church organs or the damping mechanism thud of a grand piano for instance.
Here's the spectrum of a kick drum:
Below the spectrum of the lowest note in a country song:
One thing is clear.. below the lowest note (around 35Hz) the energy drops really fast. Those levels are easily -30dB down. And that's where the driver gets in trouble. We can measure it and say how bad it is (look at Amirs klippels and see most speakers do a similar thing) but he reality is there is little to no energy there in music.
Now have a look at the distortion plot of the Clear at 94dB SPL
View attachment 100432
The distortion starts to rise below 40Hz. In music, however there isn't 94dB SPL there at all so that's why you don't hear it.. because in reality (music) the frequencies aren't there. We can measure it and even at unrealistic levels and show the results but that doesn't mean it is audible.
Enjoy your Clear.. it is an excellent headphone. Clear drivers don't break easily (the older 80 Ohm drivers, ELear, Elex etc. do) but use sensible levels and you're fine with detailed and distortion free music reproduction. Just don't try to use a tone generator to hear 20Hz at 110dB SPL. (80 Phon)
Note the 10kHz distortion spike is a measurement artifact of the used HATS due to the massive dip it has at that frequency and the % conversion.
There is no dip at 10kHz The peak around 15kHz is real though.