I hear sibilance in pretty much any ribbon design I’ve heard, PS Audio, Quad, etc being the more recent offenders. You will get fooled at shows because it is a careful choice of safe cherry picked recordings.
You can play with positioning and reduce some perception of sibilance, but, will start negating other benefits of the tweeter. You can tactfully tame it if you have front end electronics that give you some PEQ, however (i.e., when you're a straight wire w/gain purist kinda guy, sibilance can stroke you ssssweetly, lol)