Most impact:
1. speaker placement
2. room
If you can't change either of these, then I'd put my money on cabinet design. Most loudspeakers I read about that are known for not sounding boxy have creative enclosure designs and/or driver placement.
This isn't to say these speakers create a better image, but they create sound that's free of the boxes.
Shahinian, Oskar Heil, Green Mountain Audio, Tetra are four brands that come to mind.