Importance of wall behind listener?

In my audio setup, I essentially have no back walls to speak of. Behind the listener is an opening into a long hallway, an opening to a den (to the left of listening position), and an opening to the kitchen (to the right of listening position). I'm wondering if this is a hinderence in getting a truly 3D holographic soundstage representation. Other folks who have a near identical systems get phenominal soundstages, so I know it's not any of the equipment. The difference is they have a back wall vs. my no back wall, and their straight ceiling vs. my sloped celiling (peaked at center, dropping on both sides).

Any thoughts?