Zilch had dropped off a few horns from his immense collection of JBL bits, pieces and complete systems and asked me if I might be interested in doing a bit of response testing... well, yeah!
I titled this thread Does Ti Increase UHF Response? because the most surprising thing for me from this exploration was the response of an old aluminum "red wax seals intact" LE85. This plot compares a titanium JBL 2425 and 2426, and an aluminum LE85 all mounted to a JBL 2370 measured on axis at 1 meter. You can see the 2425 and 2426 have different response curves, but the LE85 to my mind was the real shiner here. Imagine what the 2421 diaphragm's response would look like!
Now, these drivers are all used, so this is not a definitive test. With a sample lot of one each I wouldn't claim that this is definitive, but it is certainly intriguing.
2425 is in Green
2426 is in Red
LE85 is in Blue