Lol I'd love to figure out what the case is with this whole vertical/horizontal argument. Any clues? What I believe as of now is that it depends on the frequency ratio between the tweeter and the woofer. Monitors with woofers that do not go that high, therefore more bottom needs to be placed horizontally.
I use M-Audio BX5a monitors which have a frequency response 56hz - 22khz. So yeh, that could pretty much be the reason. Eh?
Btw, regarding the 56hz thing, who cares? I've got a M-Audio BX10s sub, and not to mention, a super-sexy-treated room. Even had the frequency response tested and that surprised the shit out of me. Way beyond my expectations. hehe