Play a bit with this RAD parameter:
-colourgamma R G B
R, G and B can range from 0 to 1, where 0 is darkest and 1 is brightest. This parameter controls the overall brightness of your map.
Another trick you may want to try out is to let all your used textures emit some light, e.g. adding them all to the .rad file.
Or just don't add lights, you'll get full-brightness instant! But that's indeed a mappers horror...