The KAA-2 uses basically the stove stoker mechanism, and I do not think it is really designed for buck. I tried buck in my stove and ended up with a hopper fire. That may have been due to the fact that it is a direct vent unit, and Keystoker does NOT recommend buck in a direct vent stove.
The buck did burn just fine, but the larger pieces allow the draft, especialy if it is too high, to pull air down through the hopper, and the fire just follows the air (and coal) right up the grate and into the hopper. It won't burn your house down, but it sure makes a mess of things.
I can only guess but I'd believe the buck would work better in the larger Keystokers with the "classic" Keystoker stoker design (the KA-6 and larger units).