Hello George......fellow Coloradan here. In my 20 plus years hunting in Colorado, Kansas, Wyoming, Utah, and New Mexico, I have found the vast majority of keepers in these soils aren't much deeper than 6" to 8". This has held true for coin hunting old parks as well as relic hunting the Indian - Cavalry battle sites, old fort grounds, and ghost towns. The maximum age of the areas we are hunting out west is limited to about 130-150 years.
Back east hunting I have found targets as deep as 14", again soil types, and the ages of the areas go back further, perhaps as far as 250-300 years.
If you have been having good luck with your finds, and your machine at factory settings, why change what you've been doing? I think the saying is "if it ain't broke, don't fix it"!
Good luck, and Happy Hunting!
Chuck@TreasureHut