I've heard the 685s on several occasions including a one week home demo and I've also owned the 686s. MA wise I've owned the B2, BR2 and now the BX2 (see my signature).
The BX2s are blowing me away. I first heard them about 8 months ago and, in the dealer's demo room I wasn't totally convinced by them on the end of Marantz 6003 components. However 8 months on I've got that very set-up at home (newly acquired) and even though I'm only just breaking the speakers in, they are constantly impressing me with the way they draw me right into the music. They take the considerable qualities of the BR2 but improve considerably on the drawbacks. In particular, where the BR2 could sometimes sound a bit muffled in the mid-range, the BX2s have impressive clarity without ever sounding clinical. As Rick says, they punch considerably above their weight.
The 685s are a very talented speaker and I've never heard NAD amplification sound better than when coupled with them. However, I feel the BX2s give them a real run for their money and, given the lower price and less imposing design, I'd personally take the MAs over the B&Ws. The smaller 686s aren't a patch on either and suffer from overblown bass and poor clarity in the mid-range.
Ultimately you need to audition but do bear in mind my experience - the room makes a huge difference and listening at home is really the only way IMO.