As far as i understood, the speaker modes have an influence on how many bass responce the amp will produce by default.
The lowest responce is produced with "wall" - mode. Floor - mode adds a litte more bass, which is used if you put the amp on the floor with a certain distance from a wall, especially if you lean it back on the kickstand.
Free field is of use, if you place the amp on an amp stand, i.e the amp has no contact with the floor. Most often the sound has a loss of base in this case. The free field mod adds bass.
The rule is, whatever sounds good is good :D
