how to properly set gain on amplifier

This has been a big question of mine lately. I have 2 12" CVX 4 ohm with a PPI PC21400 amp. I have read how to set it with a multimeter, and that's what I did. 1400 watts X 4 ohms and then take the square root of that. It's ~74 volts. I used a 60hz test tone burnt on a CD using Audacity. I have a Pioneer AVH-P3400BH and set the volume to 30/40. I really don't know when the head unit starts to clip. I've never turned it up that far before. I also have a Sony XM-754HX 4 channel running my mids and highs and it doesn't distort. I had to turn the gain to damn near more than 3/4 of the way on my PPI to get 74 volts. I'm not sure if I did this right or not. Seems like it. Even thought the speakers can handle 1500 watts max, I don't want to distort them. I also used the dock connector in my HU to play Pandora and other music on my iPhone. The head unit has a feature that keeps all levels the same so once source isn't louder than another. I also noticed that with a different test tone at the same volume the voltage changes even though it's at the same gain level. The head unit also has a "loudness" feature. Level 1, 2, or 3. I usually keep this off, but this also changes things. Should I keep this off? Can anyone give me some feedback on this?