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The lower the ground balance number, the less depth you will go.
In order for the AT Pro to work some in highly mineralized sand/saltwater, I had to lower the ground balance to 62. Running with a 62, I was lucky to get 4-5 inches depth out of the AT Pro, that is why I went with the ATX for those conditions, and I don't ground balance the ATX unless I absolutely have to.
In normal sand and turf with the AT Pro, the auto ground balance would go down to 90-92. I have found a bit more depth without a ground balance. I now always run the AT Pro wide open without even a ground balance in the trashiest of areas, that give you the maximum power out of the machine, and silver will scream at you through iron and aluminum/gold. I found it best to cherry pick without using Discrimination.
Don't use Discrimination, don't lower Sensitivity, and don't Ground Balance, unless you absolutely have to. All three of those will kill your depth.