What exactly is a speaker box sound?

I haven't heard the effects of edge diffraction, except maybe slightly diminished imaging capability. I say that having owned Spendor SP100s twice, and they have a raised frame around the baffle.

Where I have perceived a boxy sound is in some large, older speakers that resonate at some bass notes, and give away their box volume in the process, as well as their location which spoils the stereo soundstage when it happens.

agreed. boxy sound is caused by cabinet resonance, some more some less.  

Most good designs try to minimize the diffraction effect, you'll mostly lose imaging and depth. 

I haven't heard the effects of edge diffraction

How would you know that unless you heard both a speaker with and without diffraction?

@kenjit - summed up precisely. End of conversation!