You are right, the ground must be turned off when rendering out the background, my mistake. You only nee "Final" as render option (no multipass an so on) when making the background. When doing th efinal composition, and the backround is to dark, you can either crank the 'Ambiance Intensity' on the ground surface up, which will only affect the ground, or you can turn the the 'Ambiance Intensity' under 'Global Ambiance Type' up, resulting in an overall brighter image. If the ground is to briight, you turn 'Ambiance Intensity' down. Hope this helps