Assuming the CS6 speakers are level to start, raise the front of the speakers 1/8 to 1/4 inch higher than the back of the speakers and see what that does for you regarding the sibilance. You might also need to adjust toe in. Beyond that it might be the electronics. Let me know.
PS. If you all want to hear what your Thiels can do, try these Albums titled Closer to the Music Vol's 1-5 just released on Qobuz. I'm just now on Vol 2. Captivating!