You'll need to delete/move the LOD textures/meshes each time you generate new LOD with xLODGen. It doesn't overwrite the old ones. Also there is a brightness adjustment for the textures xLODGen generates. I remember the default setting being a little dark. The way I did it was, I used Oscape for the landscape and then xLODGen for the trees and objects. I couldn't quite get the results I wanted generating landscape with xLODGen. People who are good with it can get some great LOD though. Oscape was just easier for me to use.