Originally Posted by Basic Car Audio Electronics

A piece of equipment that has a higher maximum output voltage is not necessarily going to sound better than one which is only capable of 2 volts output. The higher output will allow you to reduce the gains on your amp (or any down line signal processor) which will lower the noise floor of your system. If you are not having trouble with noise (alternator, hiss...), you may not benefit from the extra output voltage. You also need to realize that the voltage may be more than your amplifiers can handle. The extra voltage will not damage the amplifier but if the lowest sensitivity (gain setting) on your amp is 2 volts, and you drive it with anything more than 2 volts, it will cause your amp to clip*.

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Output Impedance:
Some head units (or other devices that have an audio output signal) rate the output impedance of their pre-amp lines. These specs will tell you how robust the output signal is. Since all decks have more than a zero output impedance (50-500 ohms) and all other devices have something less than an infinite input impedance (10,000-100,000 ohms), the input circuit will affect the signal from the previous piece of audio equipment. A lower output impedance (50 ohms) will be affected less than one with a higher output impedance (500 ohms). The most common (and least degrading) effect on the signal is the slight reduction in the signal level. This slight reduction is generally never noticed and is completely normal. A bigger problem appears when some piece of equipment does not have a purely resistive input circuit. This is generally from a ultrasonic filter on the input. These will cause the high frequency part of the signal to be attenuated more than the rest of the signal. Equipment with higher output impedances will have more high frequency roll off than those with a lower output impedance.