Did you try replacing the 2 drivers with the concentric tweeter? It sounds like there is a midrange/tweeter defect. Get Kef to send you the 2 drivers.
Help with kef r900 issue
I bought a pair of kef r900 speakers from kefdirect in April 2018. I enjoy them most of the time, but with some vocals, e.g., Alan Jackson, Allison Krauss and Mary Chaplin Carpenter, at nearly any level, they sometimes make a ripping, tearing or distorted sound that is hard to describe. Initially, I used a McIntosh MC202 with 200 rms, and now a Rotel RB 1590, with identical results. Initially, I used a very old Transparent Musiclink cable and now a Transparent Plus, again with identical results. I’m about to dump these unless I can figure it out since KEF has no one in the NW to repair them and they advise against paying to ship them (I agree). Before I write them off as a bad bargain, theories are solicited! Thanks