when do amps give full power

You've made a couple of points here that are not one and the same thing.  One, measuring an amplifier's output requires more than just a DMM, which Steven Kephart talked about earlier in this thread.  But setting gains is another story.  A DMM can be useful, but your hearing is the key...that is to say, by using a DMM to set gain you would be relying on published ratings, whereas when you listen for the start of the clipped tone you can't go wrong.  Sine wave test tones are a must in all cases, and they should be of professional quality and recorded at 0 db. 

We have several outstanding contributors on this forum who could put together a how-to on setting gain.  What becomes stickied is up to the admin and moderators, but I, for one, would love to see a post that we can point to as an authoritive treatment on the subject of setting gain.  Until then, it's the same ol' "search for gain".

I talked about this here, too.

realmofexcursion=good, but not the best choice unless you can't get a hold of tones on CD. Most tones DLed from the internet is in MP3 format and loses some harmonics in the tones, so it may not be a pure tone. It's close enough to use, but for precision and to keep the tones as clean as IASCA, it's best to get a CD.

If you have a DMM with oscilloscope abilities, play your music at the level you usually like to hear it. Without changing the volume, put the tone in your head unit and meter the output. What you will see is a sine wave on the screen. Increase the gain bit by bit until you see a flat section on the scope. Back down on the gain until the flat area is gone. Then go next in line (line driver, amps etc) until you get to your speakers. You're gain set.

Tip: Disconnect your speakers after you get your sound to where you like to hear it. Then set the gains.