The one advantage of mounting nearfield monitors horizontally is that they are a little lower than when mounted vertically and do not obscure the main monitors, if you have them.
Even if the nearfields are not directly blocking the sound from the main monitors, they still cause refection and diffraction effects, and the lower they are, from this point of view, the better.
However, many small studios don't have main monitors, so all of this is irrelevant. From the point of view of optimum sound quality from the nearfields, it is better to mount the monitors vertically so that the tweeters are directly above the woofers.
The reason for this is that when two loudspeaker drive units emit the same signal, as the tweeter and woofer do close to the crossover frequency, and that signal arrives at the ear via different path lengths, then there will be interference effects and some frequencies will be canceled out.
If the speakers are mounted horizontally, there is only one position between the speakers, even with precise orientation, where the engineer is equidistant from all four drive units of the stereo pair.
Better lock your head into a vice so that you stay there!
With vertical mounting then, at least for each individual speaker, both drive units are always the same distance away.
The trick now is not to slouch in your seat, because 'equal distance' only works at one height.
This is a real and audible effect. You should try moving around horizontally and vertically in front of your speakers. You will hear significant changes in tonal quality.
The only complete solution is to use dual concentric speakers such as some of the Tannoy range, where the tweeter is mounted actually WITHIN the woofer. With this type of loudspeaker, you have far greater freedom of movement as you work.
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