This article is from the Car Audio FAQ, by Ian D. Bjorhovde (ianbjor@mobileaudio.com) with numerous contributions by others.
The best way to do this is with a test tone and an oscilloscope (*Note
Oscilloscope::.) Since most people have neither item, the following
will work approximately as well.
1. Disconnect all signal inputs to the amp
2. Turn all sensitivity adjustments as low as possible
3. Turn head unit on to around 90% volume (not 100% or else
you'll have head unit distortion in there - unless you've got
a good head unit) with some music with which you're familiar,
and with EQ controls set to normal listening positions
4. Plug in one channel's input to the amp
5. Slowly turn that channel's gain up until you just start to
notice distortion on the output
6. Turn it down just a wee little bit
7. Disconnect current input
8. Repeat steps 4-7 with each input on your amp
9. Turn off head unit
10. Plug in all amp inputs, and you're done
If by some chance you do have an oscilloscope (and preferably a test
disc), you do essentially the same thing as above, except that you stop
turning the gains up when you see clipping on the outputs of the
amplifier.
Note that if you are paralleling multiple speakers on a single amp
output, you need to set the gains with all of the speakers in place,
since they will be affecting the power and distortion characteristics
of the channel as a whole.
 
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