Sub for B&W 685 Speakers

What powered subwoofer would you suggest to use with the B&W 685 speakers?

Also, would the 685s work well in a 12'x14' room?
I think the $700 sealed SVS SB12-Plus will work well in your room with the 685s. I use it in my office system with monitors about the size of the 685.

Based on the size of the driver in the 685 and its cabinet, it should do fine in your room. Of course, it all depends upon how loud you listen. I would defintely use the high pass filter in the SB12-Plus so the 685s don't have to attempt bass frequencies.
The B&W ASW608, ASW610, and ASW610XP sealed subs are part of the 600 series B&W loudspeaker line. As such, they are highly compatible with the 683s. In your relatively small room with the 683's two bass drivers (rated down to 38 hz), you may not even need a sub for music unless you really like to feel those low frequencies pound away (like I do)!! For HT, then yes you'll need a sub for those explosions and other low frequency sound effects.

For your room size, my best guess would be to go with the ASW610 with a 200W output and their new more efficient digital amp. I actually just purchased one two weeks ago as a 2nd sub in my system to balance out the bass extension and improve on room loading. But my room is 23' x 13' x 8' high with two openings out to hallways and thin carpet on a slab. A room 12' x 14' with the 683s is a different set up. That's why the 610 may be your best choice and more than you need. Just my input for you.