I used only HDRI (No lights).

I use a Half Sphere with the HDRI image plugged into it, Default lights off and the following:

Anti-Alias
Min 1
Max 3

Final Gather Rays 650 (You may need to up this value)
Min Radius 0.940 (I set this to 10% of Below)
Max Radius 9.400 (I set this to 10% of Scene Size)

Hope this is of help.

BTW: The radio Control may look like its floating a little and that's because it has rubber stops that keep it off the surface.

Cheers
Paul