If the speaker posses a relatively flat impedence curve and doesn't drop below 4 ohms then you should be fine. Of course it still depends on room size, type of music and how loud you play it.
You can always try it and if it is not enough upgrade the amps. This of course assumes you like the sonic character of the B&W's and wish to build a system around them.
I used a cj premier 11a (70 wpc) amp to drive a pair of Thiel 3.6's and it did a very good job. The Thiels are known to be a very demanding load with an impedence curve droping into the 2 range and a sensitivity of around 86db.
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