I haven't had a concentric coil in my arsenal now for a while, but if my old memory serves me correctly, DD coils are considerably better in higher mineralized ground. And they are pretty good at target separation. This thread, though, was started about coil size. Of the major detector players today, which ones have 9" as stock coils? Tesoro is gone! Deus?
9" in my opinion is just a compromise between a 11" and a 6" and will not do as well as what the 11" and 6" were designed to do. Depth and coverage with the 11" and heavy trash hunting with the 6". Hence, one could realistically argue that 3 coils are actually needed unless one is willing to sacrifice some amount of depth and some degree of target separation.
When I bought my Nox 900, the package included two coils, a 11" and 6". Not great strategy for selling more coils, but was a selling point for me to buy the 900 as those two coil sizes were near perfect for my type of hunting. On most of my sites I need all the depth I can get and some smaller size for sifting throIllugh trash.
jimmy, give the 700's stock coil a good try....I think you will be pleasantly surprised. Good luck! HH jim tn