Play with the speaker positions a little. Toe in /out & distance from the wall behind them. My 60s did this after I bumped one of them and ended up with one being toed in, and one toed out. Of course, there was overall change in sound in general as well, but the sibilance was the most obvious difference, followed by the dissappearance of any imaging at all.

Are you getting good imaging?

M- M60s/VP150/QS8s/SVS PC-Ultra/HK630 Sit down. Shut up. Listen.