The effective amplifier damping factor as seen by the loudspeaker will drop below one. Depending on the inherent low frequency resonances of the speaker, this could result in a change which is anything from inaudible to becoming very boomy. I'd probably opt for at least a 25 watt resistor and I think 2 ohms should be given a try before you try 4. In fact, if 2 ohms doesn't work, you could get another pair of 1 or 2 ohm resistors and add them in series with the first ones.