I have this new theory that roughly around 1942 the top directors began to consciously really experiment with new ideas and techniques.
http://johnkstuff.blogspot.com/2007/12/clampett-structure-clever-and.html
Even Norm McCabe did some experimenting in this period. I included the 1943 "Confusions Of A Nutsy Spy" because it is one of the first WB cartoons to really try modern things with composition and layout.1943 Experiments:
By 1946, they were still experimental to an extent but it was less conscious. By this time the directors and animators were so in-synch with each other that they just intuitively and confidently knocked out masterpieces.
Here's Jones in top form at his most confident and least conscious of trying to be clever or experimental:
Friz and McKimson were never really too experimental, they basically did their jobs and followed along with the what the trendsetters did but added their own personalities to the cartoons.
Tashlin may have tried many conscious experiments, but he never really got into a flow where everything worked smoothly together. Maybe because his mind was always on his next career and he didn't stay anywhere long enough.
http://www.davemackey.com/animation/wb/1942.html
http://www.davemackey.com/animation/wb/1946.html