Well, I did a little more research and here's how I understand it:
every +/-10 db is double or half of the percieved volume.
The 6dB difference is this:
Every halving or doubling in distance from stacks if they were set up in a straight/vertical setup will result in +/- 6dB depending on your direction of movement. The reason most concert PA stacks are set up at an angle is that it will result in a more spread out and even dB level. This means that you won't really hear a major dB difference.
Hope this cleared some things up!
Phil