I've come from the same place as you - from m50 to X2, with these being my first open back headphone. The X2 is definitely darker/warmer, the entire bass region is filled in, whereas with the m50, the bass is hitting at a much narrower range and the m50 has no mids to further muddy the sound. Obviously this is good or bad depending on what you're listening to - but I have to EQ the bass on everything that has the slightest hint of it, it's that thick on the X2. The highs on the m50 are overdone, just too grainy and artificial whereas the highs on the X2 are laid back, which works well on bass light tracks, but with bass heavy tracks, the sound is too full and suffocating. To me, the X2 is missing some harmonics, but I like how it handles the mids of a guitar much better than on the m50.
The m50, being V shaped is more versatile, but it doesn't excel at anything other than aggressive/hard hitting music, while the X2 sounds good/great for quite a bit of music, but can also sound pretty miserable on tracks that have too much low end/mids or tracks that are overly compressed. The X2 is awful for other purposes, like gaming, the bloated bass sounds so unnatural in this setting, plus that suffocating sensation is on full display. They're strictly for music. So far, I'm still trying to decide if I'm going to return them and go for something else - or just use these for the music that they can render better than the m50s, which is quite a bit(if bass is tamed).