I am thinking about getting a B&K amp to drive the fronts and use my marantz mm9000 to drive the surrounds. Is this okay or should all speakers be driven with the same brand amp?
They are both warm sounding amps so you shouldn't be worried about mixing them. I have mixed a Proceed for the mains and a B&K for center and surrounds and I love the combo. The Proceed is dynamic and accurate and well suited for the main L/R channels and the B&K is warm sounding and well suited for vocals in the center. I think this way I can get a perfect match and I have been happy so far. So mixing amps is not such a bad idea. If anything you should be worried about mixing speakers.