I hear what you are trying to say now, but I don't necessarily agree.
I don't think they add any effects, and I don't see how it's possible for a headphone to remove things from recordings other than through attenuation.
If a sound signature accentuates frequencies and adds enhancements, then that is also going to make certain recordings sound harsh.
Again I think maybe you are comparing them to what other phones have offered for so long with extreme sound signatures,
that it may be difficult to hear something that is close to neutral and so natural. Even though the audio is pleasant, they sound masked to you since you may be used to artificial enhancements.
Take that sound signature of another phone and get rid of the enhancements and flatten the curve, and suddenly it becomes natural and the harshness is almost gone.
That's how it removes harshness... The harshness is in the treble... You've already stated that the PM-1 attenuate the treble a bit... That's what makes it a natural signature rather than neutral. If a certain recording has harshness in it, a neutral phone will reproduce that harshness, a natural one will not. The harshness is not in the headphone, it's in the recording. The lack of warmth is not in the headphone, it's in the recording. There is no artificial enhancement from the headphone, there is pushing the limits in the recording. Note I keep pointing back to the recording... If you truly believe that every recording you get is perfectly mastered to sound like music, you're heavily mistaken. About 90% of modern music is recorded improperly, a neutral phone will show that to you, a natural one will not (which is why it's preferred!).
If you take a sound signature of a phone, the PM-1, and take out the enhancements (accentuated bass and midrange), you get a flat neutral headphone that will have harshness if the track has it.