I tried using CSS Compare. It seems to do exactly what you are looking for. However, it seems to have some issues with multiple selectors. The nice part is that it is command-line driven, so you can set up an automated process if you have lots of CSS files, or you can chain the CSS files together to compare them to one giant resultant SASS file. This tool will show you all the differences, including differences in the values of the classes.
What would be really nice if a tool showed you which styles overrode other styles in the same stylesheet. Many old sites have lots of baggage in this way and filtering it out would be great. Of course, an automated tool might cause issues, but at least something that generates a report like Firebug does, except for every selector and for the entire collection of CSS files, would be great. Sadly CSS Compare is not such a tool, and I don't know of one:(