backing the tone knob off the bridge humbucker just about all the way would do the trick. but bb uses a neck humbucker, or both humbuckers, and he has the varitone... however you should remember that clapton was doing that woman-tone nonsense in Cream with a single-pickup (mini-humbucker) Firebird at one point, which prompted peter green to take the neck pickup off his les paul, and both those dudes were bb-inspired and into the whole throaty bass thing.
just work at it, if it doesn't sound right, try playing it different. half of what you hear in BB's tone is his playing (pressure, vibrato, bends... a million nuances that differ in each bluesman).
