To get the best use from the RCA analog IO, Samson makes a level-matching box(around $50) that can convert 2 channels of +4 to -10 or the other way around. Its really down to using the correct tool for the job. Maybe the Mbox ix not the correct tool for your needs, or maybe the home-stereo hi-fi amp isn't the correct tool?
Maybe if you describe the ultimate goal in more detail, someone can offer a better solution