Some kind of monitor controller would be handy(unless there is a way to do it on the interface, which you never mentioned
). A company called SM Pro makes a handy little box called the Nano Patch + that could be just the ticket, and its well under 100 bucks. More money gets you more features, but this would do the trick if its all you need. Moving up the ladder, look at Samson C-Control, Mackie Big Knob, Presonus Central Station...