I have a subwoofer speaker that's rated 500 watts 4 ohms (nominal). I have an amplifier with rated 500 watts rms but requires 24 to 30 ohms speaker. Can't find such a subwoofer. Can I put two speakers with 8 ohms in series to at least give a 16 ohms? Or can I just put a resistor to increase the resistance of the 4 ohms speaker?