[YYR123]

Yeah after selecting the Playback engine you should go back to the i/o setup...

This will let you visually see what is going on...You could select default's and see where that gets you...

I personally will save the i/o setup based on my current playback device...before i change the Playback device....

Then switch engines and i/o's and then when you want to go back to the first just load saved version (import settings i believe it's called).

The version of PT that you are running should have a general default for the Mbox3.....look under import settings....and then load Mbox3 default i/o's