The manual says that because in order for the port to do its job, the speaker needs to be a minimum of one port diameter away from the wall. If you move it closer, the low end tuning changes. So does the radiation pattern but that's a different story.
The distance from boundaries affects room loading. If you want more bass, move closer; less bass, move farther. It's like EQ only acoustical rather than electronic.
There's a ton more to this though and the simple answer is that you have to play with your speakers and MLP a ton to get it right. You can start off with speakers 12 feet or more into the room or 3 feet or less into the room. Someone like Trevor may have come up with a pretty good method for getting close. I start by eye-balling the room. What I end up with sounds good and I can't know if it's optimized. But people don't like it because their room looks like crap most of the time. Actually, if they got rid of 80% of the shit they don't really need, it would all be fine!
When Bruce does his listening, he may find he loves the speaker in a particular position but not the final position. This is why I suggested he may also want to listen with room correction.
