It doesn't matter whether you use a DMM, a scope or a DD-1... you set the gain with a sine wave. For bass amps, the norm is a 40 hz tone and for mid/high amps it's a 1000 hz tone. If you have your sub box tuned particularly high , you might want to set the gain using a 50 hz tone but for the majority of car audio systems, 40 & 1000 are the best.
As for setting with a DMM... you can get pretty close if you have a true RMS meter, good electrical and an amp with accurate published RMS output.
As the DD-1 goes, there are myriad people who have tested it in real environments against a scope and it detects 1% distortion at so close to the exact same time that a good scope does that our slow assed human eyes and brains can't tell which one went off first. In short.. it has been tested against many benchmarks and it does what it says it does. And... when you use a scope, you're only looking at one frequency, so there is no difference with a scope or the DD-1 and of course you'll get a different level of clipping with a different frequency. That's called physics and again, if you're worried about it, make sure you set the gains at the frequency you will be taxing your amp(s) at the most. Same goes for a scope, a DMM or a guy who claims to have super human hearing.
With regard to the phone or whatever source you plan to use the most, set it and the bass/treble on it at whatever setting you plan to use the most and use those settings when you set the gains. And if you plan to use bass boost on the amp(s), set it before you set the gain.
As for the files you use, if you want the best protection against clipping, use file that are recorded at full level. The downside will be that when you play sources that are recorded at lower levels, you won't be able to boost them much. If you want SPL and aren't concerned with a bit of clipping, set the gains with files that are -3-5db down. That will allow you to boost poor recordings but will also introduce clipping on flat recordings at full volume.
And one last thing... if you don't have a DD-1 or a scope, set the volume level of your head unit at no more than 80% of max when setting amp gains. Either that or Google your head unit for "max volume before clipping". Also... I have an Alpine head unit that clips at 29 of 35. However, I also have a sound processor and when it's active, I have no clipping at 35. So if you have a processor, you're likely to be able to set the volume at max, assuming you haven't boosted the settings on it.
Hope that helps.
