With the exception of dual concentric and all range elements, HF is transmitted from a separate physical point in the speaker i.e. the tweeter.

That means that when U move in 3d the tweeter-ear and woofer-ear distances will vary, causing the sound 2 change due 2 different time of arrival of sound and phasing.

Standing up the speakers will allow minimum change 4 horizontal movement i.e. side-2-side.
Laying down the speakers on their sides will allow minimum change 4 vertical movement i.e. up-down.

Depending on application, the engineer will find the best placement. In a large console situation, when I keep moving up and down (sitting down and standing up) I prefer the latter.

A second factor is the large console surface, adding early reflections blending with the sound.
With the speaker standing up the distance from the console's surface is longer, time delay vs. direct sound is longer hence the phasing or "colored" fq is lower.

I prefer to keep the tweeters as close 2 the desk's surface as possible.

However: there were rooms in which I liked the speakers standing up. I guess it's because the longer the distance the less the energy of the reflected sound.

In short: try it both ways in every new room U lis10 in and c what works 4 U.

Peace,

Zooot