Duet is also the best way to listen to audio from your Mac, be it from your iTunes library, an Internet radio station, or CD/DVD playback.
To connect Duet's line outputs directly to powered monitors (Duet will not power unpowered monitors), you'll need 2 cables with 1/4" tip-ring-sleeve plugs on one end to connect to Duet's OUT-L and OUT-R jacks and the appropriate connector on the other ends to connect your monitors.