Setting External Audio Interface as default output

Hi.

I have a MacBook Pro running Mountain Lion 1.8.5 with the following audio output options:

• Built-In Output
• MOTU UltraLite Audio Interface
• Bluetooth speaker.

I have the UltraLite setup as my default for "Use this device for sound output" and "Play alerts and sound effects through this device". And it works flawless. On occassion I have to play the audio to the Bluetooth speaker. And that works great too. However if I turn off the spearker the Mac defaults back to "Built-In Output". How can I set it to default back to the UltraLite instead? (Ps-I don't mind messing with files deep in the system).

Thanks,

David