I'm not sure what your listening habits are, but I focus on the midrange the most when I listen since that's where the vocals and instruments are. The FX850 are V-shaped and as such the treble is a little louder than the lightly recessed midrange, so when vocals and instruments are at a comfortable volume for me the treble is a little in-my-face. That's where the fatigue comes in for me. These things are certainly not sibilant at all, I'll give you that, but I would never call them warm. They're pretty V-shaped relative to basically every other headphone I listen to.
I know these are Dependant on tips and I still have the original tips on these and I wear them down I only listen to them at a fairly low volumevolume. I listen to a lot rock, country, alternative music with the DX90 only, keep in mind I have only had these a few days.