To wit see Beranek, Acoustics, sec XI. The DI is not defined for a "corner" it is only defined for a "source of sound".

The 9 dB comes from only defining the sound field for a 1/8 space and using full space as a reference - not very meaningful. ANY source in that space would thus have a minimum of 9 dB of DI, making the value meaningless. Even an omni-directional source would have a DI of 9 dB by this deffinition.

The 9 dB is simply an artifact of the boundary conditions in an unrealistic situation where there is only ONE corner. Klipsch actually built a chamber like this in Hope, but I know of no other. They did it because it was the only way to test corner horns.