Hi elsewhere,
To add to JohnK's explanation, the closer you put any floorstanding speaker to the wall behind (or a side wall or both, e.g. a corner), the greater the emphasis of bass frequencies. However, as JohnK pointed out, the emphasis may be quite uneven, causing some frequencies to "boom" or seem to stand out, and others to almost disappear.
It really doesn't have anything to do with the rear ports. The same thing will occur with a sealed acoustic suspension speaker it it's moved closer to the wall behind it. As you move a sealed or ported speaker away from walls (boundaries), the bass emphasis will be reduced.
Regards,
Alan
