Thank you for your words
@nesty.
I will make the comparison using this song, because I think it has a voice, bass, a lot of definition and detail, as well as clarity and a great scene:
Before I must say that the Z3 due to its large output impedance modifies the frequency response of the headphones we connect.
To understand its effect I recommend, again, reading the following article:
http://nwavguy.blogspot.com.es/2011/02/headphone-amp-impedance.html
By this I mean that the Z3 is very dependent on the headphones we use, it may sound good or bad. An example of IEM whose sound for me is very bad with Zishan is the KZ ZS5 due to its low and irregular impedance.
However, with the KZ ZS6 it sounds quite good, it even benefits from the cut in treble that Zishan produces.
In this case I used the KZ ZS10 whose treble is not as sharp as the previous ZS.
The first thing I feel is the density of the music in Z3, while with X2 the music stretches across width and height, generating a larger and more circular scene, not as focused as in Z3.
The density of Z3 impacts in power, the voice is more intimate, while in X2 the voice is more zenith and more diluted, it melts more through the whole scene. The separation in X2 is very large, there is almost silence between the details.
In Z3 the details lack the definition, especially in mids, treble feel somewhat filthy. The bass in Z3 have a lot of presence but less depth, they are more boomy.
While in X2 the accuracy in the bass and sub bass zone is high, this zone is very contained and controlled, with a lot of depth. The bass starts and ends in X2, in Z3 it gives a feeling of overlap, they get longer.
I can not say that Z3 gives a bad sound if it is paired with the right headphones, I will never say it, and maybe the punch is its best part. But X2 has a great scene, separation, control, detail ...
and that shows without much effort.