I have tried everything from speakers exactly matching my mains to $15/pr KLHs as surrounds. I have yet to notice any difference. All that surrounds do is provide directional cues, while the overall character of the sound is completely determined by the front stage.
If I had it to do over again, I would spend far less on surrounds. $1500 for the 705s would be a total waste in that role. If you want B&W for consistency, I suggest the smallest bookshelves in the 600 series.