[OddsAre]

well, flexibility wise you would want to run a dedicated Center Section like I am doing. I don't necessarily suggest the Big Knob unless money is an issue. Even the Presonus Center Station might be a good choice since it has Digital inputs and you could use the digital optical outs from the Mac Pro. I find the line out to be noisy. But either way if you are gonna be doing a lot of mixing you want to have a dedicated Center Section for quick A/Bing, mono checks and fine control over the control room volume.

I've just always known that when Pro Tools is active, Core Audio will not allow any other program to access the interface's I/O's. I've never owned an Mbox, so not sure how it works with those.