"Speaker placement can indeed make a BIG difference-"
Oh yeah. I spend hours listening to where a new pair of speakers sound best in my room. I've owned a lot of speakers over the years and I can tell you even if a speaker designer tells you a certain placement is ok that doesn't mean its best for your room or musical tastes.
Also pay attention to your listening position. Moving your chair closer or farther from the speaker can make as big a difference as moving the speaker itself. The same is true for listening height. In the past, I've actually used an office chair so I could easily adjust the height of the chair.