I see all the decks have certain voltage ouput but is this a factor you should look into when buying a headunit or is it not much of a factor. thanks in advance.
It definately matters. Voltage is good, but impedance is also a major player in the quality of the pre-out signal. Impedance is a product of resistance and inductance, the less impedance the better because there will be less chance of inducted noise, and the signal will be much cleaner due to less inductance. That's why Eclipses preouts are so good, 8V at 55 ohm impedance, most other head unit preouts have a minimum 300 ohms. Alpines and Pioneer Premier are also pretty good, but not quite what the Eclipse is (in my opinion).