Oak,
Try pulling the sofa away from the wall about 1' to 1.5' as a test. Made a huge difference for me. The reflections off the back wall can make a mess out of the sound. If it helps, maybe you can add something to the back wall, like a heavy decorative tapestry or something. You might be getting innundated with high frequencies hitting you from both the front and the rear. They wouldn't arrive at your ears at the same time, and the wonderful human brain has a way of filtering this stuff out. The only problem is that the "filter" causes the brain to ignore the relfections, but it's not perfect, so you end up "ignoring" some of the direct sound as well as the reflected sound.
