Toning down shadow and specular on game textures

[crystallize]

I tinker with textures in old games that look good but somehow miss the mark in terms of the atmosphere with their chase for punchy visuals (HL2, currently Unreal Gold), I'm trying to tone down shadows and specular lighting drawn on diffuse textures.
Ideally I would convert the image from RGB to HSL/HSB and process every channel with its own script but can't see the real means to do that yet. Even downloaded ImageMagick but couldn't understand how to run scripts, the GUI is just missing.
Using Curve action in XNConvert seems close enough, except it does not preserve any color information in affected pixels.
Any ideas?