A Crown XLS 1002 external power amp for the front left and right channels would be powerful enough for driving the 603s. It might be a good solution to relieve the Yamaha A-870 of heavier power demands with movie soundtracks. The B & W 603s have an impedance that digs down to 3 ohms at low frequencies and the Yamaha AVR might go into protection mode when watching movies at higher SPL.
It all depends on the size of your listening room, your sitting distance from the main front speakers, and how loud you listen to your HT system. One channel of a second XLS 1002 could then be added a little later for the center channel, which most of the time requires more power than the L & R channels.