[paulo m]

If looking for a USB interface and monitor controller in the same unit, I would recommend either the Audient ID22 or the new SPL Crimson which will be out soon. These are stereo only monitor controllers though.

If going to simple monitor control SM Pro Audio offers affordable solutions in both stereo and 5.1. SPL also has some good options.

Going up on the scale, Dangerous Music, Audient, Crane Song and Grace Design offer very flexible solutions in 5.1 although a bit more expensive.

If you really need 5.1 moitoring, I would suggest buying an interface for PT and a separate monitor controller, that gives you more choices and flexibility.

If only stereo and on a budget, the first USB/Interface combos I suggested are great too.