Carefully tape a little mirror flat over each tweeter and turn the speakers so you can see yourself in each mirror equally well when you stare at it. This gets the tweeters "looking" at your ears correctly. (Then remove the mirrors!) The standard is to have the speakers at a 60-degree subtended angle, 30 degrees to each side of the listening position.
Fixing It Before the Mix (part two)
Tape Op magazine, #64, p30