I hunt with the BH Prospector (Tracker IV) with the 10" and I use to dig only on strong and repeatable signals. Well I did an air test and noticed that anything below 3 or 4 inches just barely gave a signal. With this information I started hunting with a different approach. I set my detector just how the books says Sens cranked to the right, Disc set at 12 o'clock. I keep my coil as low as I can get it to the ground. I keep it perfectly level and move SLOW. I mean I move really slow. Using this method I have just gotten a little "beep" and than nothing. I will than focus on the area for several sweeps, I usually get another little "beep". Than I switch my toggle to the center position and hit it again. If it gives a "slight beep", I move on. If it is a decent hit I dig. Once I remover about 3" of dirt, I hit it again, toggle to the right, and check my single. By using this method I have dug some finds at impressive depths. It takes time, but it does pay off. If I ignored weak singles like the book says, I would not find half the stuff I have.
Just food for thought.