I get your point. I wanted it to look clean and bright for the clean versions of the textures, so it was more based on the look I wanted (art direction) than realism. The grungier versions are darker. I didn't want them to look like Doom. One thing to note though is that the base materials for the textures were all created from photo material, steel mostly. It's not really true that metal in general has a darker backing, it mostly depends on corrosion and the level of polish applied to the metal. Polished steel is a great example.

I'm just using some parallax shader from AUM, sorry can't remember the number off hand, one of the newer ones. I think its Slins submission, but I could be wrong.