Bryston 3B cubed or 4B cubed?

Hello. I want a Bryston to power Focal 1038Be in a room 16 x 26. Mainly acoustic music, Folk, Jazz, World, moderate or less volume.  The system will be a Lavry DA10 DAC directly into the Bryston. I will make my own cables with Mogami as usual.
 I have to believe that the 3B cubed is enough amp for my application, however I wonder what differences I might experience with the 4B cubed in terms of soundstage, depth, 3D realism, etc. 
 Any input appreciated.