I'm happy with my 9's, as part of my HT. Coupled with a good subwoofer, I wouldn't consider the deeper extension of the B&W's to be a big deal. The 9's are indeed, "in your face", but for HT and R & R, I think they're great.
I've heard the 685 and 686, but not the 684. I wasn't impressed by the bookshelves, but have no idea if that would be reflective of the entire line. Since I haven't compared them, I can't say which sounds better at lower volumes.
I demoed the 9's in my home alongside the Polk RTiA 7s and liked the 9's better - obviously, since they are the ones that stayed. They're in a larg-ish room, so the extra sensitivity is a bonus. I also compared the 9's with the 7's and found the 7's to be less forward, if that's of any interest to you. If you plan to set your towers to "small" in your AVR, bass extension should not be a huge concern.