I have been forbidden by She Who Will Be Obeyed from doctoring up the walls or ceilings in any way, and given its hardwood floors, glass picture frames and windows we have a very "bright" house. (When you live in a mountain canyon, natural light is a precious commodity.)
At one point I recall asking the Forum about what I thought was an over-bright tweeter with the foregoing limitations and Skizo suggested I "put a sock in it." Still working through his double-entendre. (Actually, as the tweeter aged it mellowed and it and I are both speaking in natural tones now.)
Speaker placement, cant, toe-in/-out &etc. are highly underrated as a not-so-nuanced palliative, and is a game of inches. Labour-intensive, perhaps, but no need for trips to the hardware store or (God forbid!) visits from an interior decorator.