With your speakers only 20" from the back wall you're probably getting a lot of room reinforcement, which is not necessarily a bad thing depending on your room, equipment, and tastes. I'd first try moving them out about 4' from the wall just to see what happens, but you're likely to find that the sound separates more from the room and the speakers, and the soundstage should open up considerably(as the speakers are closer to you, you might not want as much toe-in as those B&W tweeters can get a little bright close up). One thing to watch with this is if the midrange still blends seamlessly with the treble, as with B&Ws I've found them not to be as coherent as others in a nearfield setup. After trying 4' out from the wall you can scootch them back and forth depending on what sounds better(at some points it will sound more boomy than others as you move in and out of room nodes), but at least this way you'll know what your speakers can sound like without as much influence from the room. Best of luck.
Tim
Tim