All I can do is suggest you play with the various settings and do what sounds best to you, in your listening envionment. Play around with large/small, crossover setting, extra bass. I know the general thinking is to set your speakers to small and to let the sub do the heavy lifting, but for me, on 2 channel music, I just didn't like the way it sounded. The 684's which normally have a full, rich sound, sounded really constricted when set to small. Setting them full range and enabling the sub to augment the low's seemed to sound best to me. I'm still going to play with the settings just to see if maybe going back to the small setting and lowering the crossover down to 40hz gives me the same sound.
Thinking double bass is a little too bloated sounding. Currently running small, crossover at 60hz