Hopefully Kal will chime in here. My first thought is to stick to B&W so you maintain the same midrange tonality throughout.
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I auditioned the 803 before I purchased the 802's. There is a big jump in midrange clarity with the separate Marlan head for the mid driver. The larger cabinet volume adds more weight to the bass. I love the looks of the 802 and they will blend better with your 803's.
A friend of mine has 802's in frt and 804's as rear channels in his system and it sounds reasonably coherent for a multichannel system. I am a 2 channel type person so I can't comment further as to the multichannel attributes. I do think you would find the 802 a significant improvement on the 803.