I have the GPX4800 and hunt in some mildly mineralized dirt. Dirt meter on the F75 is 2-3 bars and about 70 for GB. I hunt fairly close to power lines and have to choke my machine down but if you put the timing switch on the front cap in special and then select "sharp timing" in the digital section it will have you digging some very deep holes with all other settings in factory presets. If you encounter any EMI you will have to cut back a few settings to calm it down. In my dirt .58 cal 3 ringers and most coins are a drop in tone first and most buttons are a rising tone first. Grapeshot and fragments can be either low or high. I have only hunted a few weeks with my 4800 and I am no expert by any means but one thing for sure if the threshold is very smooth and while sweeping the coil and it changes any at all and repeats there is something in the ground. Be prepared to dig very deep holes for the faint ones unless it is a buck ball. One more thing, coins and buttons that are kinda shallow will be very loud and can even overload or sound like they are blanking.
Most of my setting are factory presets. I have to cut back on the stabilizer and gain a lot though for EMI.
Settings:
Threshold- Very faint
Search Mode-Deep
Timings-Special
Coil-DD
Tracking-Fixed
Digital section:
Backlight-Off
Battery Test-N/A
Vol Limit-12
GB type-Gen
Special-Sharp
Manual Tune-let the machine pick a channel then tune manually up or down slighlty as needed. I have to change mine often because of the area I hunt.
Motion-Very Slow
RX Gain-I have to run this very low but it still goes deep. Sometimes I am as low as 2 but never higher than 7. A smooth faint threshold is very important.
Audio-Deep unless it will not calm down then try Normal, this will help if EMI is bad.
Audio Tone-50 I like this setting but you may like it higher or lower.
Stabilizer-3 This is another setting I am cutting back on because of EMI. FP is 5 but I can never get there.
Signal-17
Target Volume-8
Tracking-Slow
Iron Reject- 5-7 I have been experimenting and you can make good target start breaking up so be careful on this one