Although I have no doubt that you're getting better at playing large fish on a tenkara rod, I do think the longer, more flexible rod helps a lot.
I wish I had a workshop or even a garage where I could do carefully controlled flex curves of the various rods. The best I've been able to do is a quick and dirty Common Cents analysis
http://www.common-cents.info/. The Common Cents system was developed to compare fly rods, and is not ideal for tenkara rods (the extremely soft tip sections of tenkara rods give nearly uniform readings for all the rods - which are nearly off the charts compared to fly rods).
One of the biggest limitations of the Common Cents system, though, is that it uses what appears to be a completely arbitrary (and completely static) calculation to compare rods. Under the system, all the rods are stressed to the point that they bend by an amount equal to 1/3 their length. When I did that with the Yamame and Amago, the reading was nearly identical - virtually the same amount of weight hanging from the tip of a horizontally fixed rod produced the 1/3 deflection in each rod (30 cents with the Yamame and 31 with the Amago).
I think in your real world tests, the large bass put a greater bend in the rod. I also suspect that the Yamame progressively gets a lot stiffer than the Amago when bent much further than the Common Cents 1/3 deflection. I haven't caught fish on either rod that come close to what you've caught. You know more about how the Yamame and Amago compare when stressed to the point of tippet failure than anyone. If you think the Amago is better for catching larger fish, I'm not going to disagree.