There are a number of rules of thumb for audio instalations, but unless you know the reason for these rules it's still quite easy to get less than optimum audio. Simply, sound waves interact inside the boxes that we use as listening rooms, much like a rock thrown into a pond. The shape of the room at various frequencies effects the sound quite a bit. So, it's important to place speakers and the listener in such a way to avoid acoustic holes or peaks. Walking around a room listening to pink noise will give one a an idea of how the sound changes with the location in a room.
The room is an extension of the speakers and sound treatment of the room are very common in pro installs. But each room is very different, so understanding some of the basics is important. There are a number of ways to tackle room sound speaker set up. The ear at tweeter level and the woofer not at 50 percent rules are used because acousticly tuning a room can be a very complicated undertaking.
Some short floor standing speakers tilt the tweeter output upward electrically (my KEF 104.2's) and many studio monitors seem to be aimed downward or have multiple tweeters. Look at these studio installtions.
http://www.mkprofessional.com/studio_installations0.htm
Nearfield monitors produce sound very near to the listeners ears, avoiding much of the room interactions. If someone is listening to speakers close up all day, they need to have different qualities than regular speakers set far away. For example the aluminum tweeters that gives the cymbals a crisp twinkle, in some floor standing speakers at home, might make your ears burn after a half hour or so listening close up. This would be a big problem, especially for someone mixing sound all day. Since high freqencies tend to be absorbed or attenuate over distance, nearfield monitors often use silk dome tweeters, which are not quite as brilliant as metallic domed tweeters.
