Some more Mudbox work. Below you can see the lighting setup. I found this setup after trying lots of different setups. I think the shadow pass is interesting on itself, so I posted it too.
As for the background: I tried many different backgrounds and I learned that most of the times it is best to follow the visual directions of the central object. In this case the central object is round, so I made the shape of the grid and the dirt in the background also round. I call this the echo-principle. :-) I will do an extensive tutorial on this in the future.