What size amplifier should be used if connecting two 4 OHM 200 watt RMS single coil woofers in series?

When connecting two 4-ohm, 200-watt RMS woofers in series, the total impedance becomes 8 ohms. To optimize performance, you should use an amplifier that can deliver at least the combined RMS power of the woofers, which is 400 watts (200 watts per woofer). Therefore, an amplifier rated for 400 watts RMS at 8 ohms would be ideal to ensure both woofers receive adequate power without risk of damage.