Oh. You're asking too much for me to rate it's quality. I'm not comfortable numbering quality.
The mids are just ever so slightly recessed when compared to the bass. The treble is smooth overall, with a slight peak at 10khz which can rarely be zingy, but it's not really an issue, and it's well in line with the mids overall. If the X1 had less bass, I'd say it's be in the realm of being neutral sounding.
The X1 is a damn good headphone, with it's weakest aspect being that the bass can sometimes be overwhelming. SOMETIMES.
I find it unfortanite that while the treble is smooth, the midrange isn't lush or full.