Importance of wall behind listener?

It is interesting to hear the different opinions on this subject. But in the end it all comes down to the room your hi-fi is located in, the orientation of hi-fi in said room, and how much money one is willing and able to spend on acoustic treatments. I actually put my 8'x11' rugs on my walls when I was still mid-fi. It was a relatively inexpensive way to aesthetically and acoustically treat a room with 10' flat ceiling's and a lot of wall space. Also, the room my hi-fi is located in was the best choice in my home, as I have a 9' pool table in my other large room. When my house was being built, I even took into consideration which of my two large rooms I would use for my hi-fi in regards to ambient noise levels outside my home during late night loud/listening sessions. My gear had to be orientated in this room a certain way due to a door in the room that leads out to the backyard. Hence, IMR (in my room), I really have no choice but to put the listening position against the rear wall. But, in playing with that seating position both fore and aft a few inches, I've noticed that it profoundly affects bass slam. I will say that there is a more enveloping soundstage with the seating postion pulled out from the rear wall, but at the expense of bass slam. And I'll admit, I'm a bit of a (tight) bass junkie. I wish I would have been into high-end when the house was being built-there are so many things that could have been done in preperation for a proper musical experience (i.e., many dedicated circuits, insulation, wallboard, etc). All that being said, and as deep as I am into high-end now, I would completely design my next (if there is a next) home around my hi-fi. ;-)

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.