VCO Mod with 18'' Mono Help

Hi BuckAu, I have just returned from a week trip to one of my favorite gold fields in S.A and was digging musket balls and snider projectiles very deep from areas that had been hammered by the amount of dig holes encountered all running 2.8 MHz on both the 2000 and 3500. I can only pick iron when it is at a shallow depth not when deep so I dig all signals I have hearing loss I dont hear high tones that may be why. Regards Ian.