you can get the sound with your mbox but it's going to take work, its going to take a great singer, and great engineering, if your serious about the song you may consider renting a class A studio with a engineer, This will let you focus on your role as a producer, at the same time you can study some tricks and ask questions to learn how to get the sound you are looking for.
I like this article on mixing R&B vocals
JAREDGosselin