Input Voltage Question

The statements are not contradictory.  In the statement "If excessive voltage is read...", the voltage they are referring to is the AC voltage you are reading at the amp's terminals, not the input voltage.

With the majority of head units, starting with the Low position will keep you from having to turn the gain excessively to match the head unit.  With your Eclipse, however, the input voltage will be higher than 2 volts so you would forego that first step and just proceed with setting the gain with the input position on High.

There is no increase or decrease in amplifier output with either of the two positions, but one position will work better than the other.  This is just a way to give the gain control two different "feels".  Ideally, you want to be able to turn the gain to some degree, for the sake of accurately meeting your target point, but not very far.

Look at it this way:  look at the gain control on the amp.  Turn it fully to the left, or to the minimum position.  With the control set to Low, that position represents 2 volts.  With the control set to High, that position represents 8 volts.  That should give you a better picture of what that control is doing.

But...as I re-read your post's last paragraph....you successfully set the gain in Low position?  Meaning you were able to turn the gain to some degree to reach your target voltage?  That indicates, if you were conducting the test properly (head unit at full clean output and pro-recorded test signal) that the input voltage was under 2 volts.  Double check your test and if it comes up good you're done.

I would say you could leave it as it is, or you can take another day and spend some time finding what you truly think is the maximum clean output of the head unit.  That should be your reference point for setting gains.  Voltage has a way of increasing dramatically when you are at the uppermost limits.

If you were able to turn the gain all the way to the halfway position while in Low, then you shouldn't go to High at all.  You would be turning it even further.  Remember that you are setting the voltage that is the extreme of what you will allow to be played...even if you never listen to it at that level.  You're setting the extreme voltage, so use the extreme deck setting (and thus maximum output voltage), just not to the point of clipping.  And finding the maximum clean output of a deck is easier said than done.  You may have to listen several times to finally arrive at the magic number.