You have to think of gain (sensitivity) like the old "squelch" dial on your CB radio. On some rainy days you had to add in a lot of squelch to remove the static from your reception. But if you added too much, you went to all static and couldn't hear anything.
Gain or Sensitivity works the same way. In Neutral soils you can crank your sensitivity to max most of the time with good results (depth). However, if the soil is moderately mineralized, you may actually lose depth by maxing out your gain.
So called “Super Tuning” is really just a load of crap. Try super tuning in red soils or black sand situations. On a good metal detector, the controls and settings work TOGETHER. Ground balance and sensitivity MUST be in tune and working together for the best depth.
Found out yesterday while searching for coins in lots of gravel and mineralized soil. I got permission to play in a private mountain camping area that's been around since the late 1800s. Thought I'd take the Vaquero along with the 5.75" widescan, as I'd figured that area could have a fair bit of mineralization... I had to turn sensitivity down much lower than I normally would, as I couldn't always get a solid target signal on fairly shallow (less than 5") coins with it up high. High sensitivity gave inconsistent, broken signals, but running sensitivity around 3-5, the signals firmed up. I guess the mineralization was just bouncing my machine's signal back at it with sens. set too high. Sort of like the "high beams in fog" example. I didn't find anything really old, but managed to add another three wheaties and a '40 Jeff nickel to my count. ^_^