FYI - I strung the SPB with Gosen Polylon SP at 62 pounds. The stiffness of this racquet is 70. The Head I strung with the same string at 60/61 and the stiffness is rated at 69. Seems pretty close. I'm not sure what dynamic is at play here, but the SPB definitely has the ability to launch the forehand and serve with more power if I swing as hard as I can.
The one thing I do notice is that the Head has a slightly more dense string pattern than the SPB.
Prince Speedport Black with Gosen Polyquest 17 @ 61 lbs (4.5 USTA)