Two likely issues. One is the vertical dispersion of the tweeter is different for your standing and sitting positions. You might try changing the tilt of the speakers if possible.
The other is the reflected sounds probably differ between standing and sitting. Look for differences in the first reflection surfaces (left, right, up, down and even behind) between standing and sitting. Strong reflections will muddy the image. Changing speaker position or adding sound absorbing materials at critical points may help.
You'll probably have to experiment to find a solution, but it will probably be centered around a factor noted above.
The other is the reflected sounds probably differ between standing and sitting. Look for differences in the first reflection surfaces (left, right, up, down and even behind) between standing and sitting. Strong reflections will muddy the image. Changing speaker position or adding sound absorbing materials at critical points may help.
You'll probably have to experiment to find a solution, but it will probably be centered around a factor noted above.