When we want to use our Macbook Pro Retina to listen to youtube vids and other stuff like spotify etc we change the audio under "system settings" and then the "sound" (speaker icon) otherwise it just plays in the built in speakers which are always turned down to "0" so what happens if you chose your audiointerface under the sound icon (in preferences) pressing the apple icon.
Sorry I have the swedish OSX so I can't say what it is called in your iMac but you probaly know what I mean
Best Regards
Christopher