I do rememeber reading how each reflective surfaces adds 3db to the bass responce. (Notice it is not a power gain, just a shift in Hz responce). So, relative to your seating posistion, 3db for floor, 3bd for backwall**, 3bd for sidewall**)
** if speaker is "too close" to back or sidewall.
** if speaker is "too close" to back or sidewall.