although I am not sure what your soil conditions are like there...if you're finding clad at 4-5" deep, then the area has seen some fill added for sure.
This however does not mean that the oldies are not still down there. Hopefully they are still within reach.
I have a couple places I hunt where there has been up to 8" of fill added. I am still able to hit the oldies, but you have to use one heck of a slow sweep speed, and really overlap your swings.
If you're using an Explorer, you should be able to suck the oldies out from under that fill. Unless of course there is a foot or more, then you're likely in trouble
Not sure what you have for coils, but if you own a WOT coil, I'd throw it on. This would get you a little extra depth...and by the sounds of it, the trash at the site is minimal, so you should be okay. If you don't have a WOT, stick with the stock coil obviously.
If you want some settings to try out, shoot me an e-mail

I don't want to give away all my secrets here *haha*
One thing to try, which many don't consider. Find a spot on the field that sees little use...get out your digger and dig a wide plug...dig down a good foot and look at the walls of the hole for stratification. Again, not sure of your soil there...but normally you would dig down a couple inches through top soil (dark and rich), then into the sub soil (probably 4-6" of lighter soil). If you see this pattern, chances are it's undisturbed. My guess is it's not though. If you're hitting dark soil, or worse, marbled soil consistantly down past 6 inches...it's fill. Anyhow, if you get that, keep digging down until you hit a consistant soil type/layer. That will be the original soil layer.
Wess