I've had 2 different sets of C4 fins, Mustang 30 and Falcon 30, both the T350 carbon, not the T700. My Mustangs were just about the perfect stiffness. Not too stiff, but still great at coming up from depth. Now the Falcon 30 that I had were stiff as boards, not comfortable at all. I've heard people call Falcons up to 40 stiffness like wet noodles, and I was just curious if maybe I'm a sissy and need a softer blade or maybe I just had a set that was a little stiffer than normal.