I don't know how much a pre amp is going to help you in a live situation. I mean, you can run a nice mic pre, but if its going through a bad PA then it's still going to sound bad. Also, mixing boards have their own pre's so not sure why you would need another one really unless you were going for some really pro sounding stuff.
For recording I think having an outboard nice tube pre amp is necessary, but for live, I'm not too sure if it is worth it.