OP. slav is right.
THere's one thing to be aware of though.
If you have an interface with a built in preamp and you buy a new standalone preamp, then yes you'd plug the new pre into the line input of your interface.
(trs to trs cable)
In some interfaces the line input completely bypasses the built in pre (which is what you want), but on others it's just accepts line level and still goes through the preamp circuitry, which is not good. (A pad before the preamp i guess)
The simplest way to know is to plug any trs cable into a line input, then turn the gain up and see if it gets louder.
If it does, you're still going through the built in pre and getting all the lovely noise that comes with it!
Sadly, I believe the mbox doesn't completely bypass it's internal preamps.
That's not to say an outboard pre wouldn't sound better with your mbox. It just won't sound its best.