Wow. This thread is supposed to dispel myths?
There is a lot of misinformation here!
Ideal source impedance is zero ohms (someone mentioned this). However, the best source units are 50 ohms with most in the 200-1k range.
A typical amplifier input impedance (Z) is 10k-50k ohm per channel. Do the math: with 200 ohm source impedance and 10k input impedance, you can drive 10k/200= 50 channels with no signal amplitude loss. But with 1k output Z and 10k input Z, you can only drive 10 channels before signal loss. For the SPL crowd with 30 amplifiers...this matters! They need line drivers. A line driver isn't supposed to make everything louder, it is simply a current amplifier for line level voltage. It keeps the output voltage up so that it can drive hundreds of amplifier inputs!
So where do HV output headunits actually make a difference?
It depends on how crappy your amp is.
Think about it this way: You are simply transferring the gain increase to another place. If your headunit is quieter than your amplifier at full gain, then get that HV output and turn the amplifier gain down. Vice versa? Save your pennies and keep that 1V output HU.