You have two main choices for DC step-down:
Linear Regulator: Inexpensive. Throws away excess voltage as heat.
DC to DC Converter: More expensive. More efficient.
You didn't say what the input voltage is (your 5A supply). You didn't say what the power requirement on the three output lines (4.5V, 6V and 9V) are. You did say 1.6A but is that one EACH or all combined?