I know this doesn't answer your question but what I've read is that for realism you don't necessarily want to use the same exact map in each channel. Sometimes bump maps are blurred a bit from the original (I think so that the light/dark transitions aren't so drastic) and the reflection map can be hand-tweaked, etc. www.dvgarage.com has a lot to say about texture maps and Digital Texturing and Painting is an interesting read.