The 684, with dual 5" mid/woofers, goes down to only 72 Hz; but the 686, with a single 5" driver and a much smaller cabinet, goes down to 62 Hz? Does this sound right?
As they presented those specs, it is confusing.
684 dual 5" drivers in a floor standing cabinet
Frequency range -6 dB at 30 Hz and at 50 kHz
Frequency 72 Hz – 22 kHz ±3 dB on reference axis
Sensitivity 87 dB
686 single 5" driver in stand mount cabinet
Frequency range -6 dB at 54 Hz and at 50 kHz
Frequency 62 Hz – 22 kHz ±3 dB on reference axis
Sensitivity (84) 85 dB
The 684 is more sensitive, and should sound like it produces more bass than the 686. Both are likely to be bass shy without a subwoofer.
This would be much simpler to understand if they provided a frequency vs. loudness graph.
A much bigger potential problem is their use of a 4 kHz crossover point between the kevlar mid woofers and the tweeter. In previous B&W 600 series models this has been known to cause audible problems of harshness in the lower treble range and listener fatigue, depending on the musical material. I don't know if these new models deal with that successfully.