I previously paired the 803D with a pair of Marantz SM17 bi-amped. The 60W per channel SM17 worked surprisingly well. The Onkyo M504 will work fine as long you like how it sounds with the B&W. Try it first with one before buying another to bi-amp. See my previous posts for my experience with B&W and bi-amping.
Also, if you can afford it, I suggest you consider the 803D rather than the 803S. The 803D is a nice tonally balanced loudspeaker with reasonable clarity but more forgiving than the latest 803Diamond.
Also, if you can afford it, I suggest you consider the 803D rather than the 803S. The 803D is a nice tonally balanced loudspeaker with reasonable clarity but more forgiving than the latest 803Diamond.