Yeah, I think that is a very tough scene, because of its simplicity.

BTW, I've been playing around a little more and have another idea for you. I think I may be trying to be too clever by half. Go to the regular Render Settings. Switch to the  Manual Tab and check the Indirect Light box. Then go back to the Auto tab and use those presets. It looks after a short study as though they wind up setting reasonable values over in the Dimension 3D panel and are a lot less hassle. I think you will have to get up to the setting that has at least one raytrace bounce going to really get any IDL, but it's worth a try.

Whichway