The deepest silver quarter I ever found was a 42 in nice black dirt in some woods buried under some roots I had to chop through but even then it wasn't all that deep...maybe about 5-6".
With my F70 I actually was in 2F tones at the time, DE, disc at 4, threshold at +1 or 2, sense in the 90's.
All my other silver quarters and pretty much all the clad ones too rarely ever got deeper than that and I have dug them in 3 states.
Heck, even an 1898 Barber quarter I found had to be hiding in the to dirt for about 100 years wasn't much more than 2" deep.
I have dug deeper coins though, an IH coin spill at about 9", some silver dimes about 7-8", a couple of deep nickels including a worn V at about 8".
Most were found using 4H tones.
Nowadays I mostly use all metal or monotone with low disc at 1 or 0 for their unmasking abilities and also for some great depth.