I had the opportunity to compare the LCD-XC and the HE400i side by side. I can tell you with certainty, the LCD-XC is a far more detailed headphone. It was obvious that it had greater clarity and detail throughout the range.
When I first bought my LCD-XC, I auditioned the LCD-2, LCD-X and LCD-3 in stores (LCD-4 was not available at the time). I had the money for any of them and preferred the sound of the LCD-XC over the others. Which was good for me since I was biased towards a closed headphone anyways. I don't see much a point in buying a private listening device if it is not private. The store clerk also agreed with me and said LCD-XC was his personal favourite to. I won't state any particular reason for it on this forum as it was a subjective listening experience. But I can say it is at least as detaiedl as the LCD-3.
Becareful reading opinions on the LCD-XC model. They updated their drivers in 2016 and drastically changed the tunning on the LCD-XC. It became much more balanced tuning, leaning off the heavy bass. Still really nice kick in the bass, but not so much overtaking the rest of the range.
Audeze's support is some of the best. If you have to warranty the drivers, they will renew the warranty on them for another 3 years. It is also transferable.