I got into Univ of Ill. in 1999 with a 570 GMAT but did not go.... So I think you are fine in the 600s. The competition for MAcc is not that tough. And to be honest, I really do not see any value. I am also a CPA and did the public accounting thing. But since you started a business and if it is doing well, you may have a good chance to get into a good school even if you nail a mid 600s.
I am not sure about your background, but if you are a female, your chances will even be better.