2v or 4v receiver?
 

I want to get an aftermarket head unit for my 2014 Z w/ factory non-bose speakers. Which option would be best for me if I don't want to replace the factory speakers, 2v or 4v in regards to preamp? Will either hurt the factory speakers?

12V.

If you mean Ω, then you need to match the power amp's impedance to that of the speakers. I can't recall off the top of my head what the impedance is for the base speakers. Someone else will probably know.

The voltage I mentioned is for the preamp.

Hi JCarter,

1 volt should be enough to drive your power amplifier to full power. BUT, and a big but, is what is the sensitivity of your power amplifier input? Most power amps have a pretty wide range with the input sensitivity adjustment, about 3-4 volts down to 300 millivolts (3-4 with level of power amp to minimum sensitivity, and 300 millivolts with power amp input level set at maximum). It's best to start out with the receiver at a nominal level say if your receiver will go from 0 to 50, then set the receiver at say 25 and then turn the power amp (with clean music -CD preferred) up until you hear distortion, then leave the power amp adjusted and then turn down the receiver, then go to an "Auxiliary" input and listen to the background hiss, if it's really loud then turn down the power amp a little. It's a gradual process. You should be able to find a happy medium adjustment for your setup without resorting to extremes on receiver level or power amp level.
But, if you have 4V preamp output on your receiver, then you'll get more voltage with less turns of the receiver level knob. Before "6V" or greater receivers became popular, everyone somehow got by with 1-2 volt outputs and the same power amps we're using now (but in older packages, haha). 95% of AB power amplifiers (most of what's out there) are still using the same switcher IC in the 12v DC to DC convertor, and the same output transistors and just repackaged them in new chassis.
Now, the mini amps that use Class D switching are neat little packages, but basically have a DC to DC convertor and Class D outputs.

I would recommend you ask BigAudioFanatic, I'm sure he has all the popular receiver/amplifier combinations settings memorized.

Get 4v pre outs for headroom. You did not mention anything about your amps so not enough info. You should swap speakers at some point.

Get 4 volt if you can, lot less chance of noise interference the higher the voltage.

Absolutely... The higher the better. And your pre-amp voltage has no bearing on your speakers whatsoever, It just feeds an audio signal to another source (Amplifier) which in turn feeds your speakers. The Base model speakers, by the way are 4ohm, and can handle quite a fair amount of power, and sound good doing it.
Fountainheads method of setting your amplifier gains is pretty rudimentary, but will work, the best option for setting your gains is to figure out your voltage with ohms law, and a multimeter.