Ok I finally managed to activate "high gain" mode with my custom iems and spent last couple of hours with them and the MX4 Pro, so I can add more impressions.
The sound to me is a little brighter compared to the MX4 pro, even if it is not quite as dry/thin as with the DX100: bad recordings are a little more obvious, but it stays soft compared to the ibasso, or the more violent Cube C30.
And this is probably where I recognize "Meizu's sound": even though the MX4 Pro has a sabre audio chip, the sound generally remains smooth and natural.
Thanks to the more convincing soundstage, details are presented in a far better way (like echos going from left to right side, lasting longer etc etc), and you loose less micro details than with the Pro MX3.
I also find the highs to be noticeably better, emotionally speaking.
My honest opinion: the sound is indeed better, the Pro handles headphones a lot better, and will certainly allow you to enjoy your music more than the MX3.
Now, if I had the choice, I'd have probably saved the money for something else, as the MX3 filled all my needs on the go, and as a smartphone.
If it had the DX100's soundstage, I probably would not feel the need to invest in another home source.
You can find the 64GB version for 440 euros now, on aliexpress.
Hope that helps