Using a DMM seems most likely to be pointless to me:
When audiophiles arbitrarily choose* a test tone's attenuation level, whether it be 0dB, -3dB, -6dB, etc., it's really just a guess, or an estimate ... Then they're instructed to use a DMM to take a very accurate measurement of a system that includes a major variable that was a guess.
* even though they've done their best to choose a test tone level
... which is probably the main reason amp manufacturers don't tell you to use a DMM; From my Alpine PDX-5 manual:
Set the PDX-5 input gain to the minimum position. Using a dynamic CD as a source, increase the head unit volume until the output distorts. Then, reduce the volume 1 step (or until the output is no longer distorted). Now, increase the amplifier gain until the sound from the speakers becomes distorted. Reduce the gain slightly so the sound is no longer distorted to achieve the optimum gain setting.
From my Kicker MX700.5 manual:
The input gain control is not a volume control. It matches the output of the source unit to the input level of the amplifier. Turn the source unit up to about 3/4 volume (if the source unit goes to 30, turn it to 25). Next, slowly turn (clockwise) the gain on the amplifier up until you can hear audible distortion, then turn it down a little. Adjust AMP 1, then proceed to AMP 2 and finally the Subwoofer Channel input gain control.
I dunno ... It all seems like a "Make your best guess, and hope you don't burn it up" to me. I just wanna be able to crank it and know I'm not hurting anything.