It is not possible to load RTAS plug-ins into auxiliary input channels in versions of Pro Tools earlier than 6.7. In 6.7 or later you can play RTAS instrument plug-ins through auxiliary input channels. If you load an RTAS insturment plug-in into an insert slot then you will be able to load RTAS effect plug-ins in subsequent slots, however you cannot load RTAS effect plug-ins without first loading an instrument. This means it is not possible to process audio sent to an aux input track using Liquid Mix. Users who wish to do this need to upgrade to Pro Tools 7.