i'm not sure how it works when a track is remixed, but i would imaging the artist has to grant permission to the remixer for an official release. Were any of these rarer mixes actually sanctioned by Underworld? If not, how can they be sold, as there are copyright issues? Of course, if a mix is done for private use or for a dj set, that's one thing, but as some of these were pressed up, presumably for sale, well....hmmm.
