Depends on what you want to do. Your post reads as though you are attempting to mix down several channels to 2-track during recording, and replaying a 2-track output at the same time.
My guess is that what you actually want to do is record raw individual tracks and mix them down in Pro Tools. You may also be wanting to re-play tracks already recorded to headphones. One usual way of doing this is to take the direct outs (1-8) of the 1604 to the line inputs of your interface. However, the MBox 2 pro has only 4 analogue inputs (2 line and 2 mic/inst), so you would be limited to recording 4 separate channels in this way. That may be enough for you.
You don't say why you want to take the MBox outputs back through the mixer. For 2-track monitoring, you are much better off using the headphone outs or line outs of the MBox to drive headphones or active monitors. In this way, you also avoid the chance of a feedback loop.
On the other hand, if you want to mix several replay tracks down to 1 or 2-track before feeding to the monitors, then you do have to use the mixer. In this case, the recommendation is to split the mixer logically into two: channels 1-4 (say) for recording, using direct outs 1-4 connected to the analogue inputs of the MBox. Mbox outs 1-8 connected to line inputs 5-12 of the mixer. Mixer main outputs to headphones or whatever you are using for monitoring. Be sure to keep faders 1-4 at zero (not 0dB!) the whole time.