Momma always said, "Boxy is as boxy does".
There will be a limit to how much you can reduce boxiness by repositioning your speakers.
You see, boxiness is caused primarily by internal resonances that re-radiate through the drivers and/or through the port; by internal resonances that re-radiate through the cabinet walls; by panel resonances in the cabinet walls themselves; and by diffraction, primarily at the cabinet edge.
All of these result in unnatural colorations. Am I implying that there are natural colorations? Yes! For example, the ears are quite forgiving of even-order harmonic distortion and comb filter effects, as these are naturally occuring phenonema. But boxy resonances are not, and the ear has a low tolerance for them.
When listening at normal volumes, imagine putting your ear right up to the drivers. There's just as much sound coming off the back side of the drivers as the front, and all that goes into the box. Unless it is completely absorbed, it eventually comes back out to haunt you. Back when I built my own speakers, I used transmission lines because that enclosure type does the best job of getting rid of coloration from the backwave of the driver. I even did it on tweeters - I popped the back off of tweeters with a rear chamber and loaded them into a short, tightly packed transmission line. I also experimented with very rigid panels and various panel damping techniques, including mass-loading and even sandwich enclosures with a layer of plumber's putty in the middle.
There are several aftermarket products that can reduce boxiness. You might experiment with adding DeFlex panels inside the boxes. You might try felt around the edge of the front baffle - but this can also suck the life out of the sound, so listen for that. If you have ported speakers, you might try tightly packing the port with drinking straws cut to about 5% shorter than the length of the port (this will preserve the original tuning but reduce toilet-paper-roll midrange colorations).
Unfortunately, it is expensive to build a box that doesn't sound like a box. Fortunately, there are speakers out there that don't use boxes, or that make limited use of them, and one of these might make sense for you. Maggies, InnerSounds, Martin Logans, Sound Labs, Quads, and in discontinued lines there's used Apogees and Acoustats. I've probably left some out.
If you find a good deal on a pair of used Gradient Revolutions or anything by Paragon (now sadly out of business), these speakers are about lowest priced box speakers I know of that don't sound like boxes.
