+1 to what Mark says.
It is important to note that once you get serious about audio in a business sense, professional studio monitors are the way to go. A huge improvement they have over passive speakers with a receiver/amp is that they are already powered and that part of the signal chain is omitted. It is not only solving a reliability issue for the hardware, but for the process itself and the ability to rule out strange issues in the process.
It should be noted as JohnK mentioned, that usually 2 sets of "monitors" are used as a stink check. The guys I have talked with usually go with Mackie for the A speaker and Yamaha for the B. Basically a good one and a meh one. Different brands are a good idea to see if the mix plays well across a variety of loudspeakers.
If you are into this kind of stuff then "Gearslutz" forum is for you.
http://www.gearslutz.com/board/
