I dont know what to tell you except turn the brakes up high, even with a loose spool you shouldnt be backlashing. My gen 2 sx is a backlash machine unless I baby it or turn up the brakes 3/4 of the way up. I keep the spool tension super loose, just enough to take out side to side play in it.
Set the mags on the stx and see, im not to familiar with the centrifugal brakes on those models, but try messing with those also.