Small cabinet, narrow baffle, rounded front edges.
C.. if you seal the inside of a wooden cabinet with Cascade V-Bloc that is a water based product designed to be absorbed into the wood pores and drys like concrete. Also use non ferrous fasteners for all drivers and crossovers,replace all resistors with non inductive type as sold by North Creek and others. Direct couple the speakers to the floor with points that are designed to collect, focus,and discharge resonant energy to a larger mass or ground.Tom
1. speaker placement
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.
If you email me then I can briefly describe how using the virtual room treatments from Machina Dynamica may be just what you need. Great WAF because can be hidden in plain sight, or barely noticeable when in position.
I have listened to before-and-after results of true speaker builder-modifiers, and the results working outside the cabinets with Machina Dynamica can be as effective, sometimes even better, than what is heard after a great deal of hard work by the builder-modifiers.
Certainly recommendable for those listeners who are all thumbs.
Additionally, inexpensive Big Fat Black Dots (speaker footers) from Herbiesaudiolab.com enable many speakers to sound their best.
Finally, after years of DIY speaker building, at least one experienced speaker builder I have encountered feels "the magic is in the cross-over".