Just listen to a solo voice which on panel speakers can occupy the whole stage (perhaps what is were the term 'big mouth' is most appropriate). On these speakers, to compensate for this manufacturers add a ribbon tweeter to bring the high frequencies into better focus.
Then there are omni directional speakers which by design rely on reflective sound to create a large multi dimensional sound stage. They do work well with big orchestral pieces in making them sound like a live performance, both of which have reduced resolution and specificity (unless you sit in the orchestra pit regularly).
Then there are 'box speakers' which can, if properly designed, set up, driven, etc, accurately portray the music as recorded, if that is what is important to you. Unfortunately a lot of recordings really have no 'height' element in the pits/grooves. The effort is great and the rewards can be small.
I think that is why so many folks love omni, panel, and electrical speakers. With a few notable exceptions, the sense of a full sound stage is always present and rarely faithful to the recording.
In setting up box speakers to get maximum soundstage reproduction I think you are best served by minimizing the effects of reflective surfaces and the effects of too much bass or treble response (too much bass masks high frequency information, too much upper mid range/HF information gives bright unnatural sound not heard in real life. (Unfortunately many folks think this upper frequency information is evidence of 'resolution' capabilities of their systems, a thought encouraged by a lot of manufacturers/retailers, etc)
I've overstayed my welcome by now. Find a copy of Depth of Image! :-)