Since 2-ohms rating would mean that the voice coils offer lesser resistance, the output sound would be louder compared to 4-ohms impedance. However, you must note that minimal resistance means more power consumption which eventually leads to lower quality in the bass. As for 4-ohm resistance in your subwoofer, it may not be loud but the bass it produces would be clean and distortion-free. To get the best results, we suggest that you try connecting multiple subwoofers of different impedance measures in different series and parallel combinations, whatever suits you.