I'm growing white widow at the moment. I've read other growers feeding these guys without issue. I on the other hand have had nothing but problems since moving to flower.
Before I started using canna, I had the same questions. Every other nute line adjusts acourding to where you are in the grow. Cannas site has great info on this. I also emailed them about it, and basicly they said when you use their line, it is designed so the plant and coco work together and the coco only releases what the plant needs. They also told me that the coco controls the PH so that it is perfect through the grow. I did weekly 1:1.5 soil tests, and this proved true. Rootzone PH during veg was a constant 6 and during flowering 5.8. I think I remember reading on their site that the starting water should be between .2 and .3, and most of that should be Cal/mag/Fe. If the Cal is off it will interfere with N and P uptake.
What I don't understand about canna A+B is why they always have to be used together. For example, I currently have an N tox problem, but I can't dial back the N without dialing back the P and K. What if my plant is sensitive to too much N but will gladly take more P and K? According to Canna, the plants get the N whether you like it or not.