8 years later reply..April, 2019, I bought a Fisher 15" DD and tried it on my Gold Bug Pro. It works. It detected a flat new quarter 12" deep in the ground. I set the lines on the detector knobs straight up, ground bal in all metal mode. Adj knob for slight tone. The tone barely changed when over the quarter, no number displayed. The tone tried to become a pop when I manually adjusted the ground balance +- 10. The Fisher 15" white DD coil is for 13 or 14 khz, possibly? The Gold Bug Pro runs at 19khz. Eh, I'll try it. 15" coil is VERY heavy! Thinking of dragging it behind me or a harness..yes, I'm after gold nuggets, trying a different approach.
Forgot....These 15" DD coils are $119 and change at Kellyco.. not $249!!!