Importance of wall behind listener?

I also have a similar configuration, but with a 9" flat ceiling a 12'rear opening in the center of the rear wall (an entry/foyer followed by a 12' hallway). I found that a Janis 15" Subwoofer placed in the rear corner added a significant demensional improvement.
This unit is upward fired, and has a variable output. But once you have it set it blends very well.I have no experience with diffusion or other sound treatments. Other responces are very positve, but I am concerned cosmetics.

A few random ramblings and this is an issue I have wondered about for along time. I think it is interesting that so many live performances have the best seats in the front third and, in larger venues, the back is very open. In all the indoor orchestral concert halls this is the case. Bass is reinforced by a sealed room, but that also makes it harder to control. How many of us have openings to the rooms our equipment is in? I have found that I can compensate for the bass with large subwoofer. My next purchase will be for a second sub to even out the bass frequencies. The high and mid range is usually quite good without a back wall, since there is no concern about reflections from the back. The responses so far are interesting!

In my experience, there is a trade off between placement for bass performance & soundstage. Usually, if you place the speakers nearer the front wall or sit near the back wall, you get better bass but the soundstage suffers.

I would suggest that a better way is to chose speakers with the bass performance for the size of room even when it is placed in the optimum position for the best soundstage.
In my room, the rear of my speakers are 4 feet from the front wall and I sit 5 feet in front of the back wall and on the better recorded CDs, I can hear an all enveloping soundstage which extends to ambience & echos coming from behind my listening position. In a friend's place, where his back wall is further back still, the effect is even better and you really get the illusion of sitting in a concert hall.

Hope this helps.

I had a similar problem in a large unbounded space and resolved it to my satisfaction without consulting Rives or purchasing acoustic treatments. After some experimentation, the extended space behind the listen has become irrelevant. There is now a deep convincing soundstage and wonderful bass performance, in balance with the rest of the spectrum.

I got this result by covering the stud wall behind the speakers with 3/4" plywood. I believe this helped rigidify the wall and boost the bass energy. It also seems to reflect the sound toward the listener which in this situation is favorable. You can experiment for yourself by leaning some sheets of finish plywood against the speaker wall. When I did that I could hear the difference immediately, so I mounted them to the wall. If it doesn't work for you, try the plywood somewhere else, or use it for some other project.

You could also try experimenting with a partial room divider, shelving, or piece of furniture at various distances behind your listening position. My hunch is you'd want something more absorptive than reflective in this position. Don't spend any money on treatments until you've gotten some direction from mockup arrangements of items you already have. Go by ear and have fun figuring it out.