frequency response - B&W vs. Monitor Audio

If you're getting a powered sub, then it does not matter whatsoever whether your bookshelves go down to 42Hz or 49Hz. More important is how the speakers perform to your liking, in any case.

As for the high frequencies, it's really irrelevant whether a speaker goes up to 22kHz or 30kHz since human hearing maxes out at 20kHz, and for most individuals, much lower. So unless you're playing SACDs of dog whistles for your pet's enjoyment, this is also a non-issue.

Get the MAs if you like them better than the 685s, but not because how high and low you think they can reach, especially in this case.

The MA BR line is their entry line and IMO inferior to the B&W 685s you arleady own. If you like the MA sound, I would consider the new MA RX2. These have an 8 inch woofer in a bookshelf cabinet and they rock. Really high value for money. Also, they would be fanstastic with a NAD C326. The Cambridge stuff will probably be too bright with MA speakers. NAD is a much better choice.

Jaxwired your point is well taken regarding the BR2s, and if I had done my homework before buying the B&Ws I may well have ended up with the MA RX2s.
I think I am going to buy the powered sub and upgrade my amp as well; with a powered sub, would the CA still seem as bright? I like the CA's dual speaker channels, enables me to purchase new speakers (B&W 683 or better) later and basically have a 4.1 system, on my way to building the perfect 5.1 surround system for the living room.
Also, the CA 650 has 25 more watts than the NAD, but it does not have main-amp-in connections. Still no one has posted regarding the idea of high-passing the 685s - good idea or not?