To be honest I do not believe its an issue with room treatments. I dont think treatments can help my soundstage but maybe I am wrong.
Room treatments can do a lot for the sound stage...heavy broadband treatment of the rear wall behind the listening position can deepen the sound stage significantly - vocals will become more articulate and tangible too. Perhaps you need to re-examine some of your beliefs and experiment with some broadband absorbers to reduce reverberations. G'luck.