Unfortunately, Digi and most users of 8.0.4b (b is for beta btw
) have been having huge problems with setting the primary device like in XP. I myself have been having problems too and have had to resort to using the output of laptop into the input of the Mbox. Its not the best but it works for now.
I will mention that 8.0.4b is a beta and therefore will have bugs/issues/problems and I know that Digi are working hard to get this resolved with the audio drivers. Please bear with them, they are working hard
James