hi every body i am a new x-terra 70 owner so far so good . i have the standard 9" coil that came with the detector, but would like to have another coil. i live in n.w iowa. myGB numbers are inthe 40 to 50 range not much mineralization. i want to get a 6" coil, i basically coin shoot most areas are pretty trashy.which coil would be the best choice. thank you.