I'm not familiar with the P6, but B&W generally need lots of power. Despite the power rating on the Marantz, I don't think you can drive them to their full potential.
There are some very good full-range speakers listed on Audiogon - in your price range - that I think would make a better match.
There are some very good full-range speakers listed on Audiogon - in your price range - that I think would make a better match.