In Mac OS X, you have the option to route all system audio through a pair of virtual channels in the Console for additional level control, as well as the ability to process the signal through UAD-2 plug-ins in realtime.
Why might you want to do this? One possible use would be to EQ the soundtrack of a movie you are watching on the internet. You could also route iTunes playback through the cue outputs to be included in a headphone mix, if you were playing along to another track for example. Another benefit is that once you have routed signal through the virtual channels, you can also easily record from those channels into your DAW software.
How can we set this up? First, you'll need to set the Apollo as your output device for system audio. You can quickly do this by Option+Clicking on the speaker icon in the Menu Bar at the top right corner of your screen and selecting the Apollo from the Output Device list.
Setting the Apollo as the output device for system audio via the speaker icon in the Menu Bar
Next, head to Audio MIDI Setup, which is found in the Utilities folder within your Applications folder. Once open, navigate to the Audio Devices window shown below - if the Audio Devices window isn't shown right away, you can access it by going to Menu Bar > Window > "Show Audio Devices" while the Audio MIDI Setup application is in focus. In the left hand column, select the "Universal Audio Apollo" driver. Select the Output tab, then hit "Configure Speakers" in the bottom right.
Navigating the "Audio Devices" window of Audio MIDI Setup
Now that the "Configure Speakers" window is open, select the output channel numbers that correspond to the pair of Virtual Console channels that you wish to route to. For most applications, choosing "Stereo" and setting the Left Front and Right Front will be sufficient. However, if you are using applications that support Multichannel audio (such as QuickTime or DVD Player) you will also want to choose "Multichannel", set the format to "Stereo" and then proceed to configure the Left Front and Right Front speakers there as well.
Configuring the Speakers, Stereo Tab
Configuring the Speakers, Multichannel Tab
Note that the channels corresponding to each output channel number will vary depending on which Apollo you are using and how you have configured the system. To see which channels correspond to which output channel numbers, check the Outputs column of the Core Audio panel in Console Settings (Console > Settings > Core Audio).
"Virtual 1/2" shown in the Outputs column on the Core Audio panel
Now that your system audio has been routed to a pair of Virtual Console channels you can begin applying UAD plug-ins to these channels, routing the signal to cue outputs, and applying whatever other creative routing you can dream up. To record the audio from the Virtual channels back into your DAW, just create a new stereo audio track in the DAW and set the inputs to correspond to the Virtual channels you routed your system audio to (i.e., Virtual 1/2 in the example above).