THe picture you have is the correct way to setup an 2-channel amplifier with two 4 ohm subwoofers. Although this is simple, you can achieve more power by hooking up both subwoofers in parallel to achieve a 2-ohm load to a mono- amplifier. Think of ohms as being resistance, the higher the amount, the harder the electricity has to work to achieve the same power. If you wire the subwoofers in parallel, you can have this resistance to 2-ohms. Not only this but for most mono amplifiers, they have two speaker outputs, so hooking them subwoofers up would be no different then your picture describes, except you would be getting more power. Long story short is if you have a 2-channel amplifier, this is the correct way to hook it up but, the better setup is a mono amp with both subwoofers in parallel. Theoretically, half the resistance, twice the power.
