Using Test CD’s

Yes.  If you are playing a regular music source the voltage will jump all over the place.  You won't be able to fix it at a spot.  Typically, use a 50 Hz tone for the sub and 1000  Hz for the mids.  Disconnect the speakers while reading voltage at the speaker terminals.

It's incredible how touchy the gain becomes when you are at the top few volts when you're setting the gain.  Just a touch of the screwdriver will put it into a higher (or lower) level.  This is something that just can't be experienced when tuning by ear.  After doing it this way, it makes you realize that "when you hear distortion, turn back the gain a little" really means just the slightest touch.

Tegpilot, use Ohm's Law to figure it out.

To find voltage, E = SQR( P X R ).  R is the ohm load, P is the watts you want to achieve.  (When deciding on what formula of Ohm's Law to use, first determing what two known factors you have.)

For example, I set my gains recently on a mono amp with a sub that has a 200 RMS rating.  So I wanted 200 watts (as long as the amp will achieve that, which it could).  The sub is a DVC 4 ohm, wired in parallel, so the load is 2 ohms.  Using the formula, 200 watts X 2 ohms = 400.  Use your calculator to find the square root, which is 20.  So 20 is the voltage I wanted to set the gain at.