I agree it is a problem, but I believe you can get good bass from those speakers when only 18 inches from the back wall. The measurement to the back wall is just one measurement. There are several others to be played with. You ought to also think about the other measurements to closest boundaries - floor, side wall, ceiling, and the opposite wall. Take those measurements accurately and look for mathematical relationships, such as is the 18 inches to the back wall the same as or twice or half the measurement from the bass driver to the floor, or to the side wall. Using these measurements to shift the bass nodes around the room can even be calculated by available computer programs if your room is cuboid (is that the right word?). If all else fails, get radical about the seating position.
What I am saying is that I have had to cope with this 18 inches to the back wall issue with several speakers before, and been able to find something that works in the bass. The issue that is often much harder to deal with is that 18 inches is barely enough to avoid the soundstage collapsing due to reflections of higher frequencies than the bass. This requires damping of reflections between and behind the speaker, and when the speakers are this close to the wall, making sure there is nothing between those speakers.
