I understand your setup. The reflective objects will reflect the background whether it's an background image or HDRI image. I was asking because the plane (which is not very reflective) is very dark as though it was not receiving light. I have tried to create an HDRI image and it looked great with reflective objects. But once I added non-reflective objects, it showed me that my HDRI was not generating any light. That's why I was asking you to put in non-reflective objects to see how much light you are really getting from the HDRI image.