I don't mean to slam the X, it really does have merits, but I've decided that it has an irritating sibilance region which is a coloration, not an accuracy. It's fatiguing to my ears, and harsh. I also own the HE-500 and that has a more natural treble to me, although not as detailed as the X, but to me it demonstrates the difference between "bright without irritation" and "bright with irritation."
Really curious about what components you had hooked up to the LCD-X to produce the sibilance. Not casting any aspersions on your kit - I found gear selection had an impact on sibilance with my LCD-X.
I did manage to produce a small about of sibilance on my LCD-X when I had it hooked up to my Simaudio 100D DAC and WA6 amp. When I swapped this out for my Auralic Vega/Taurus rig the sibilance disappeared. I tried very hard to reproduce it and couldn't. I blasted Shirley Bassey doing Goldfinger - lots of high vocals and high brass treble - several times. Not a hint of sibilance anywhere.