I find that I get better depth with my Excelerator 5" DD "Hockey Puck" in Prospector Mode. In the really trashy sites that I hunt mostly I have to put up with a cacophony of sounds and noise but I like to dig most solid positive signals anyway. I run gain as high as possible, SAT as high as stability will allow and sweep REAL slow. A deep signal may look like trash and very "iffy", but slow narrow sweeps will usually "lock on" to a good target. I watch VDI stability and listen for short faint sounds. Damp ground helps a lot, too.
I don't know of any way to tell good targets from trash in the lower positive range. Many good items read exactly the same as foil, tabs and other trash, so to be sure I guess you have to dig them all. Actually, I'm rather glad it's that way because most people won't bother. That leaves a lot of good stuff, especially gold, still out there waiting for someone willing to put in the back-breaking labor to pull it out.
Marc