To me, there's a huge difference in power level between the 63 and 70 RA. Obviously, the 63 is much lower powered, and the result is more control and fewer stray balls flying long. I can really blast a serve or even a service return with a full swing and the ball stays in the court 90% of the time, even if I don't strike the ball cleanly.
Stiffness-wise, I would actually say that the 63 FEELS stiffer than the 70 for some odd reason. It's softer on the arm, but stiffer feeling when you hit the ball. It's a very wierd sensation. All I know is that the 70 hurts my elbow slightly and the 63 doesn't hurt my elbow at all. So there must be SOME difference in stiffness. But when I hit the ball, the sensation is that the 63 feels stiffer.
I had a good string setup with the 70 version... it didn't feel very string-sensitive at all. But the 63 is a different story. All strings that I've tried so far feel stiff in the frame. I've even tried proportional stringing with little success. Next up is a good soft multi to see if that can make a difference.
Anyone run across a good string for the 63 RA frames?