With a few more hours of run in, the upper mids' edginess has refined a bit. It's still bright but not sibilant, per se.. just impossibly crisp. Crisp, tight, synthetic snare hits you commonly hear from 80's pop records have an articulate snap to them that would put some BA's to shame.
I'm listening to Michael Jackson's "Thriller".. and the the FXT90 is simply devouring it. The superb instrument separation, impressive clarity, and pinpoint imaging is really something else for a $150 phone. These have really excellent control & dynamics from top to bottom too. I get the sense music feels like it can "breathe deep" when run through the JVC. In complex pieces, the phones never feel strained or struggle to keep all sounds in their place.
Leaving out the Sony's freakishly wide soundstage, the FXT-90 is everything the EX600 wish it could be.