Not sure if anyone has mentioned this yet but you could try elevating the speakers. To see if moving them in the vertical plane has any effect. Even if you can’t keep them like this it might help determine if it’s the effect of a vertical room mode. You could also try angling the speakers up to see if that helps clear up some of the nasal sound.
I find that my front soundstage sounds much better, more ambient and more precise, when my front speakers are elevated so that the midrange drivers rather than the tweeters are at ear level.