Complete 8 reps on the preacher bench with the EZ bar. Then transition straight into seated dumbbell curls, working one arm at a time, progressively increasing the number of reps from 1 rep each side, 2 reps each side, 3 reps each side, and then finally 4 reps each side. The progressive building of reps, supersetted after the preacher curls, ensures that the muscle fibers are recruited and hit hard.
The angle of the preacher bench puts the arm in a forward arm flexion position, which places an emphasis on biceps brachii. As such, the brachialis is active throughout the exercise. Preacher curls are much harder to complete than standing curls and place stress on the muscle at both ends and throughout the entire range of movement. Also, they prevent you from pulling your arms into extension to gain additional assistance from the long head of the biceps or assistance from the delts.