Accuracy is equally as important if not more so, than speed or power.
If the cue ball strikes the front ball off to one side or the other, a significant amount of whatever power the cue ball has is lost to the cue ball running around the table. Or even flying off the table. The ability to hit the head ball full is absolutely paramount to a strong break.
I recommend trying a break cue or shaft that has LD, or low deflection technologies. The low cue ball squirt properties will help you to hit the front ball square which will transfer much more power to the rack. And, it'll control the cue ball much better too.