I don't see why that wouldn't work for in wall speakers. Subs have their own amps. The center is a bit of a mismatch though. While Phils are the way to go, I would get the matching B&W center. In walls are not the path to great sound IMO. Fine for surrounds and background music, but not mains.
The B&Ws are 88dB sensitive and -6dB at 45 Hz, so they shouldn't be terribly hard to drive. Magnolia, even though they are probably right in this case, are the wrong people to ask questions. Marantz may be overstating that it wouldn't be enough, and I'd ultimately probably go bigger, but I think that one will be fine unless you expect very loud listening levels.