It's almost a common sense relationship here....

First it has nothing to do with increasing or decreasing noise....it's more about taking a clean signal from the head, then keeping it clean to the amplifier.

Think about it....if I have a head that outputs 0.5V and a second that outputs 4 volts of signal. Both will theoretically pick up the same level of noise along the signal chain. The 4 volt signal's amplitude is 8 times higher then the 0.5v signal. Therefore, the noise will be less significant on the 4V line then the 0.5v line.

Also, as voltage increases, you can decrease the gain on the amplifier....and we all know that's a good thing. The lower the gain the better!

Heres some additional reading for you to prove it's not heresay...

http://www.eatel.net/~amptech/elecdisc/gaincon2.htm