I currently own both the X and MX4. I fell in love with the X when I originally bought them, then swooped on the MX4 over Christmas at 50% off. For MSRP there's no way I could justify replacing the X with the MX4. In Canada however, where the cheapest I could find the X was with the creator's package at $1600CAD, then $2200 to my door for the MX4 + Woo Audio stand was a fantastic deal. Regarding sound, they are incredibly close. I find the MX4 noticeably easier to listen to without correction, whereas I'd highly recommend the X be used along side an EQ or Sonarworks. Beyond that, they're so similar that on sound alone, I sometimes find myself slightly preferring the X. I'll be selling the X though. Why? Because to me at least, the X with less compromise. Lighter, sturdier build, premium feel, sleeker visual design, pelican case, etc. I love both, even the X had a few minor annoyances that I was happy to live with, they're all missing with the MX4. As such, I rarely find myself reaching for the X, simply because the MX4 is the same but comfier nb. Had I not scored such a deal on the MX4, I'd have zero desire to part with my X.