1. Assuming your head unit actually could put out 50Watts RMS (very doubtful), then yes, they would only be running at about a quarter of their max power (assuming that's 220W RMS). RMS (since you don't know what ohms are, I assume you don't know what RMS means either) is an acronym for "root mean squared". Basically, it's a useful measurement that electric engineers use to determine the useful power or voltage, etc. of a device. You get this by taking the peak power/voltage, etc. and dividing by sqrt(2). Blah.

2. Ohms are a measurement of electrical resistance. Named after the guy that discovered it. You may have heard of ohms law: V=IR (Voltage = Current * Resistance). So, the resistance of a wire, or a resistor, creates a voltage drop along the wire. In car audio, an ohm rating usually represents the nominal impedance (basically resistance in AC) of the speaker. Most speakers are 4 and 8 ohms, subwoofers are usually 2 or 4 ohms, sometimes 3 or 1. When an amp is listed at a certain ohm rating, it means, when driving a load (a resistance, or speaker, is considered a load) it can put out a certain amount of power. As the load becomes more efficient (smaller ohm rating), the more power an amplifier can produce (except for certain designs, like the ones from JL Audio). So, if you have a 4ohm speaker, check the power rating of the amp for that impedance. If the the power is listed for 2ohms though, and you need the power for 4 ohms, just divide the listed power by 2.

3. I pretty much explained what that means above. They are simply saying the same thing twice. It makes it easier on you to figure out how much power you will get for your particular application (i.e. subwoofer). That amplifier is not very powerful. Unless you get a 2ohm subwoofer, you will only get 125watts out of it.

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