[JohnM]

The problem of being able to change the default output for a Java app (i.e. REW) on the Mac but not the input is a "feature" of Apple's JRE, unfortunately.

Your sound card would seem to have the right features to work under XP, the problem there is more likely a setting on the card's mixer software than anything to do with REW - just leave REW set on default input and output and use the card's own software to find the right setting. If the card has a choice of driver types, make sure you load WDM drivers. Focus on getting any input to the card recognised, once that is sorted out it should become straightforward. Make sure monitor is turned off to prevent feedback loops.