You lose at least 30% ground depth when comparing air test to actual ground depth.
I believe the reason you get a louder report in Disc mode vs. AM is in Disc mode they increased the gain to amp to sound up--"high gain circuitry".
In AM mode target detection, depth of target the audio is somewhat linear. Deep targets may barely change the threshold audio, then in disc mode give a louder report.
I use both modes to determine target size and depth, or you can determine if the target is trash.
Now if I remember correctly the Cib and the Vaq in disc mode the target report doesn't have much of a variance between shallow or deep targets. Its loud. This is where all metal mode comes into play.
Tesoro detectors I have had, noticed air testing on the average difference between disc and AM mode was about 1" to 1 1/2". Others about 2-3" difference.
Going to positive, negative in your GB you will either gain or lose some depth depending upon target.