Connect a resistor in series with the speaker. Fix the resistor either at the output terminal of the amplifier or at the input of the speakers. Connecting a resistor in series does not literally increase the speaker’s impedance; instead, it raises the total impedance of the amplifier driving the speaker experiences. Note that combining resistance in series only requires you to add up the impedance together to get the total impedance felt in the circuit.
Install matching transformers to the speaker’s circuit to increase its apparent load. Examples of this kind of transformers are ZEROs. Connect the two output terminals of the transformer to the corresponding terminals on the speakers. Also, connect the two input terminals of the transformer parallel to the circuit from the amplifier with the positive terminals joined to the positive of the amp and the transformer negative to the amp negative. These transformers are capable of multiplying impedance to appear as optimum loads so that the speaker’s load matches that of the amplifier output.
Connect another speaker in series with the one whose load you wish to increase. This method is almost similar to using a resistor instead. By doing this, you increase the total impedance of the amplifier and the system will work properly and safely.