Tascam US-600 isn't working, works with GarageBand.

Anybody else use the Tascam US-600 input and have issues in MainStage? I use it with GarageBand and have zero problems, but I get nothing in MainStage. In MainStage's preferences it acknowledges that it's there and I can select it as an input, but I get no sound at all. Any help or suggestions would be greatly appreciated. Thanks!