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.
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.