Playing with positioning hasn’t helped in my case, although I’m constrained due to system being located in living room. For example, I can pull speakers no further out than 3.5 feet (from front baffle). I’ve played with toe-in extensively and noticed changes in soundstage but not sibi
Unfortunately, I cannot treat room. Sibilance has only recently become an issue for me, however. With previous systems, I never heard it.
Stored away somewhere in your house, might you have a dac that came with a PEQ feature, like an RME dac or other? Or can the audio nerd who lives down the street loan you his RME? If so, put a wide Q dip (a small amount) between ~4kish and ~8kish hz and see if that helps.