Biggest difference between b&w 600 and CM series?

I am looking at the 684's and in that same price point the CM1's and am wondering what to be looking for. granted i am in a smaller apartment and don't have the budget for huge dedicated amps and such. i own a pioneer vsx-52 now and am planning on bi amping it. My main question is would it be beneficial to look toward the CM1 and plan on saving more to use these as rears or in the future CM7's or 9's in the fronts, because untill recently i was pretty set on the 684's. any suggestions??
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