When you rebuild with lightmass you should be getting bounce lighting.
It looks like there is actually a subtle amount of red bounce light hitting the floor in your second image. It is more obvious if you scroll down the window so that the actual red cube itself is not visible.
You could try selecting your directional light and increasing the indirectlightingscale and/or saturation. Also you may want to reduce the intensity of the EnvironmentColor or AmbientCubemap since it could be washing out the bounce lighting by adding the opposite color.