I hv both rackets. My tour 660, is modified to 13.2oz w/ most of the lead wt. is at 12 and 6 0'clock positions in the hoop, strung @61lbs w/ ultra tour 17g and dunlop titanium17g, the tour 660 has a head leather grip added/increased to approx. 4 5/8 grip now. If u still hv yr tour 660, I highly recommend u try this set-up (massive kick serve and 'plow effect' w/ great control on the grd. strokes ( i use 2-hands-backhand w/ this racket). I prefer, heavier rackets w/ lower balance points, approx. 100sqin.
The PC600, is much more 'demanding' for me esp. at 12.5-13oz, the balance point seems too high and adding a larger grip has not helped. I find the 660 is best 4 me (flexibility/ comfort/power/control). BTW, the unmodified 660, is a very low power racket. All the head prestige rackets r very elbow friendly.