This seems to be a popular question, as I'm addressing the same issue and considering the same alternatives.
My conclusion is the 7B-ST, rather than the paired 4B-ST. B&W told me straight that my original choice: B&K 250wpc monoblocks, would be a respectable solution at modest listening levels. The B&K monoblocks are built to handle peak dynamic loads. However, that the next step would be to provide robust power to the LF: The speaker's impedance profile changes driver by driver, HF --> LF. My solution is then, the 7B-ST with an internal bi-wire: probably XLO.
My conclusion is the 7B-ST, rather than the paired 4B-ST. B&W told me straight that my original choice: B&K 250wpc monoblocks, would be a respectable solution at modest listening levels. The B&K monoblocks are built to handle peak dynamic loads. However, that the next step would be to provide robust power to the LF: The speaker's impedance profile changes driver by driver, HF --> LF. My solution is then, the 7B-ST with an internal bi-wire: probably XLO.