the MPB's speakers are of very good quality, unless they are not loud enough for you (maybe the room is very big) i just say stick with it. Also all that 5.1 o 7.1 audio system is crap. The fact of the matter is that the more speakers you have (unless you are standing still, not moving in the "hot spot") the more interference will be (the more crappy the audio will be) thats why after all that surge for 3.1, 5.1 and last 7.1 audio system with huge bass and all the surround sound crap, we are going back to the traditional 2 speakers system. Therefore if you really need a bit more power try to get a set that has the fewest speakers, if it can be just one even better (usually they come in pairs so finding a set of 1 might be though.)
And btw for the guys that said BOSE is ****, i dont agree, they are pretty good, but not $300+ good.