As I mentioned before, Steve, you are a good observer.
I do notice that new growth canes tend to be more vigorous than old growth ones in most roses, regardless of class.
As I said above, new wood is always more efficient than old wood. That is because it can move moisture and nutrients through the canes much easier than old wood. In a way, you can think of old wood as having an form of arteriosclerosis ... a hardening of the arteries.
Even with your HTs that you think are not vigorous enough, now that you are watering them more, take one cane out to give it a kick start towards better performance. You don't need to do it to all of them, but you can experiment and see if it makes a difference. I'd pick one that has been around for a while ...
