This has probably been discussed many times in the past but I can't recall seeing anything about it. When I first start detecting, I auto GB my X-70 and then manually GB. I usually GB with the coil I last used which could be either a concentric or a DD. Does it make any difference what coil I GB with. I use what ever GB numbers to determine the mineralization and what coil (concentric or DD) I should use for the area. Any info. would be appreciated.