What will be the difference between having 2V vs 4 or 5V amp output leads? Is it even noticeable?

you just have to make sure your amp can handle the input voltage your putting in to it..lets say your rca out put on your stereo is 5 volts and your amp input can only handle 2 volts then you would be over powering the amp,,the volume will turn up to fast and the music will be distorted..

You want to match the stereo out put with the amp's input level..

The higher the RCA output voltage the better. The greater the signal coming into the amplifier the more efficient and cooler the amp will run.

As stated, the higher the output of the deck, the better. The "gain" knob on an amplifier is not a power adjustment, but rather a sensitivity adjustment. As you turn the gain up, you make the input more sensitive to the low voltage audio coming from the source.

As you turn the gain up, the sensitivy magnifies EVERYTHING, not just the audio, but also any noise in the signal, random RF, etc. You're always best served to have the gain as low as possible. You can achieve this with higher input voltages. If you do some searching, there are actually boxes out there that will take a lower input voltage and amplify its output (look up the Audio Control Matrix).

By using lower gain levels, and higher RCA voltage outputs, you can generally achieve a louder and cleaner sounding system than turning the gain up with lower voltage RCA outputs.

After the responses and not knowing it was an amp related issue, I then double checked my amps owners manuals. They are capable of 200mV -8V input. (JL Audio). For clarification, a call to the JL tech guy resulted in the exact same comments as above.... More noise possible. Thanks!

JL makes some nice stuff. I would have said I'm sure they will accept at least 5v just by hearing their name. All of the big names with quality products in the industry make amps that will accept 4V+ these days.

Get 4V minumum inputs. That way if you have split an RCA with Y adapters, you will still have enough signal to the amps.