Stimulators

Technically it's a slightly smaller chance with a stimulator due to the <10% bomb rate, but intentionally shooting for a decent amount above average isn't very realistic because of the low chances. You'll probably want to rely on the effects of an advanced black/dark crystal for that, which, in my experience, I've seen to have equal chances of each result (-3 through +3 or -5 through +5), unlike natural variation. Black/dark crystals' effects are applied after those of stimulators though, so you may still come out with below average even with a stimulator.