Hello there, hope you are all doing awesome! :)
As the title says really.
I read this https://www.presonus.com/learn/technical-articles/studio-monitor-placement and checked the manual for the 3.5/4.5/3.5bt/4.5bt and found nowhere on the PreSonos website that says they can be used on their side/horizontally or are designed for such use. The positioning article linked above does mentions the E5 and E44/E66 can be used that way but that is it.
If you google "presonos eris 4.5 horizontal positioning" you can find what appears to be some official multi-lingual .pdf manual for the Eris 4.5 that state they can be used on their side, and how to arrange them correctly, but I do not know if they are old versions of manuals and no longer relevant as nothing on the PreSonus site indicates this and that manual cannot be found on this site, and horizontally is the way whatever monitors I get will be used due to my setup.
Thanks in advance :)
Edit, something else I found on the Amazon listing for the E44/E66 is:
Horizontal or Vertical
Many MTM configurations must be arrayed vertically, rather than horizontally, because vertical placement provides a clear, interference-free midrange and treble reproduction to either side, expanding the sweet spot. By bringing the midrange drivers closer together and raising the high-frequency driver, the E44 and E66 are able to perform optimally in both orientations. So, like all Eris speakers, the E44 and E66 let you position your speaker the way that works best for you.
which seems to denote all Eris speakers are designed to use both ways? But I cannot find that on the PreSonus site anywhere.