used to think heavy weights with 6 reps was the key to adding mass...
however, seems like the greats like Ronnie and Arnold would use "lighter" weights (not 10 pound dumbells!) and do more reps (greater than 10 per set). they would still lift heavy ass weight, for instance, if ronnie can press a 220 lb dumbell for 6 reps, he would instead do 180 lbs for 12 reps. Still very heavy, but "lighter" than usual so he can do more reps.
seems like arnold believed in that too, feeling like a better pump built more muscle than using very heavy weight with lower reps...