I had an XLT & and almost never dug a coin below 4" as I only dug signals that the display said were probable coins. The visual ID below 4" or so had numbers jumping around so much it was hard to guess what it might be. I sent the detector to the factory. They put on a new coil, but it still had this problem. I tried hundreds of different settings, but could not use settings anything near those suggested by others. So I sold it after finding $1,200+ in shallow clad and few older coins.
Mostly detected in new parks & knowing the age of them helps. Found 11 silvers in a 12 acre 1963 park & 3 silvers in a 3 acre 1964 park. These places have more clad, too, compared to the heavily detected older parks.
One XLT user posted about using the non-motion all metal mode for extreme depth. I did an air test, a Kennedy Half got about 10" in motion disc, but about 18" in all metal non-motion. The only clue to a possible coin would be in pinpoint, if it seemed coin-sized & round.
I got a Garrett AT Pro now. Not long after I had the ground balance set properly, I got a loud beep that indicated dime on every sweep. It was a clad dime down 7". I was impressed as my soil is moderately mineralized & was dry & hard. The clad dime was found in a heavily detected park made about 1973-74. Best wishes, George (MN)