One week later and my initial impressions are overall the same: fantastic highs, better mids than FX700 (i.e. not as recessed), lovely sound. Bass is indeed emphasised, a little too much even for my taste but very clear and does not bleed in the mids. However, depending on one's source and music genre, I can see how it can steal the show and obscure a bit the rest of the range. Keep in mind however that I nitpick here.
Mid-bass bump aside, bass is impressive - goes extremely low and is distortion free, has shattering impact and completes well the rest of the sound. A significant challenger from this point of view to XBA-H3. Which in my book is quite an achievement, I'm yet to hear better lows in an IEM than what XBA-H3 can deliver when properly tuned. As XBA-H3, FX850 greatly benefits from cutting a few dB from 110-120Hz as the rest of the spectrum has more space to shine
What JVC should be admired for with FX700/FX850 is that they managed to get a transparent sound and highs that extend to the sky, yet at no time (to my ears at least) the sound is harsh or sibilant - which, with all the praises that I have for IE800 and craving for its out of this world highs, Sennheiser did not manage to do.
Add to all this the "secret woody sauce" that gives the signature JVC timbre to their sound and we have a winner. Not the most accurate IEMs I listened to given the bass emphasis, but not that far either. And even arguably one of the king of universals such as IE800 does require eq-ing (to my ears at least), so I don't see why applying the same to FX850 would make them less worthy.
All in all, I love the FX850.