D2X-XL supports two different lighting methods: The standard method, where all light information is pre-computed by DLE (or DMB2), and OpenGL style lighting, where D2X-XL computes a level's illumination when the level is loaded, using formulas similar to OpenGL.
With standard lighting, DLE computes each vertices' brightness and color depending on all light sources that affect that vertex. You can influence a vertices' brightness by adjusting light range and overall brightness in the light tool, and also by changing the brightness of any texture that is used in a level (that means you can even turn textures into light sources that were not initially emitting light, and vice versa). This gives you great control over how a level is lit.
With OpenGL style lighting, D2X-XL uses the brightness and color values of a level's light sources to compute the light in the entire level. The only way the level designer can influence the result of this operation is by adjusting the brightness values of the light sources in the level.
Be aware of the fact that as soon as you chose to have D2X-XL use colored lights for non D2X-XL levels (e.g. Descent 2's main campaign or any other standard level), D2X-XL will use the OpenGL style lighting method to compute a level's static lighting (that is the light emitted by stationary, indestructible lights). D2X-XL will also do that if told to use colored light in a D2X-XL that doesn't contain any light sources that have been assigned a color by the level designer.
You can adjust a texture's brightness in the texture tool. Select a side bearing the texture the brightness of which you want to change. If the texture is an overlay, make sure to uncheck the "use 1st" and check the "use 2nd" check box. Then adjust the texture's brightness with the slider or directly enter a value. This function is particularly handy to decrease the over the top brightness of some animated panels. Note that any such adjustments are fully compatible to all Descent 2 program versions as they directly influence standard lighting.
To light a level, it is therefore recommendable to adjust light placement and brightness for OpenGL style lighting and make sure the level looks as desired with that lighting method. Once you're satisfied with that, adjust DLE's standard lighting parameters until you are also satisfied with the mines standard lighting.