Ohms are a measure of resistance. An amplifier, in this case the one in the head unit, will be stable down to a threshold resistance, often a exponential of two. But their power is rated at a certain resistance, even if they are capable of going lower. So your headunit is rated at 52 watts (peak) per channel if each channel has a resistance of 4 ohms. That's just where it is rated. Somewhere in the specifications, it should have a stability threshhold. It will say stable at 2 ohms, 4ohms, etc. If you crank the power with too little resistance, the amp will burn up. Add more resistance, and the power output (actually the amount of power that gets converted to sound) reduces, meaning the system is less efficient. So you want speakers wit hthe lowest resistance that the amplifier can handle. Lower resistance = louder output, up to the point where too little resistance can damage the amp.
With the model number, we should be able to find the lowest stable resistance rating of your head unit, but if I had to bet, I'd say it's 4 ohms per channel. :tunes: