Using a 0db tone makes 2 things very certain. 1. you will never clip the output stage of your amp.. 2. you will never see full power out of your amp. Like others have said most music has peaks near -3 and -6 average on many songs is as low as -10.. -3 corresponds to 1/2th power and -6 is 1/4th full power -10 is like 1/10th (not exactly on that one but closer) . Why buy a 1000 watt amp if for most of the song it's only going to put out 100 watts and at peaks put out 500?
Yes setting gains at -3 you may clip your amp on peaks, but it's also giving you 2x the power everywhere that your not clipping, meaning musical averages woudl be 200 watts and as high as 500 and the full 1000 watts.. Despite what many "gurus" on here claim, clipping is NOT always a terrible thing. Mild clipping for brief periods is NOT hard on an amp. Idiots how don't know how to use a volume knob and their brain/ears give clipping a bad name. Very hard constant clipping breaks an amp.. Light clipping on short sections of music allow you to play things loud and dynamically..
If you are moderately intelligent, gain setting does NOT require a DMM.. To be honest those who claim it requires on and say there is no other way to do it are simply showing their lack of knowledge about actual audio.. They use gain setting to match power. ALA, subs can handle 500 watts so use gains to make sure thats the max they recieve, mids can take 200, etc... That's not what gains are for, gains are to match VOLUME!!!! 200 watts on a midrange produces very different SPL's than 500 on a sub. Or maybe it might be the same, that's what gains are there for! =Meaning even though you have different speakers with different efficiencies and power on each playing at the same time, being turned up at the same time by the same volume knob, each speaker will get louder by the correct amounts to stay balanced. It's individual to each setup as different amp and speaker combos need different gain settings to sound correct. Obviously, this means you are adjusting power, but the end result is for the correct volume, not for the correct "RMS". Best way to do that is to use your ears or an RTA, but not a DMM, power isnt' what matters, it's output balance..... You can tell when a sub amp is clipping and set gains by ear.. Play a very bass heavy song at the highest HU volume you plan on using with sub levels at moderate gain on your HU (leave a little bit more gain to go up in the front of the car). Slowly turn up the gain on the amp in the trunk until A. it's loud "enough" or B. the sub hits mechanical limits OR stops getting louder/moving more.. once a or B happens turn it down just a hair and BOOM, your sub gains are good. You'll notice any time you adust the gain knob at a certain point things stop getting louder, at that point either your sub has hit's it's limits or the amp has, either way that's max volume on the actual loudest stuff you'll listen to.. that's what counts in the real world for daily. The extra little bit you left your gains to go up by on your HU allow you to add bass to songs that aren't super bass heavy.. People that set gains using 0db with everything maxed out, what the heck do you do when you want to add bass to poorly recorded music?
If your going to burp a car for 3 seconds, yeah, a DMM gain setting method can help, maybe that's where this obsession started.. Also, I guess if all you listen to is 0db recorded rap all day at max volume like Jeezy and Gucci, ect, then yeah, a DMM works as good as anything there too. However, if your listening to normal pop music, any country, jazz, basically ANYTHING and can use your head and ears when playing the real heavy stuff, just set gains by ear.. Get use from your equipment..