Standing vs. Seated-Big Difference in Bass???

Bass Note waves are very long and so nodes and anti nodes are created that mean the bass is diminished or absent in certain spots or enhanced or even doubled in others. You seem to be seated in an anti-node point. Standing up fixes it, but other possibilities are moving your seating position forward or back. Other parameters to play with are your speaker positions. The good news is that you get good bass somewhere which means you have to just work with your room, not ditch either it or your speakers. Sometimes a few inches of movement in each speaker plus your seating position is enough to shift nodes and antinodes quite significantly, so you just need to experiment. Don't bother with the speaker spikes till you get the amount of bass right. If nothing quite works, try setting your speakers up against another wall, or setting them up slightly off-set in the room.

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.