My understanding is that all Explorer coils are Double D. The stock coil, Pro coil and SEF are all Double D coils. I don't think there are concentric coils made for the Explorer?
The only reason I can think of that a Double D coil would not do well in a particular soil is due to lots of mineralization giving falsing and nulls. This may be due to the increased ground coverage that the Double D coil gives you over a concentric... so it seems less stable and more chattery because it is picking up more "targets"? I know there are plenty of people using Explorers in Virginia soil, so I'm thinking it may have more to do with settings than the actual soil.
I have found that the Explorer has very good target separation, but you need to go slow. If you're sweeping it like a weed whacker, you're not going to find much and you'll get falsing.
I have the Pro coil and the depth is impressive. More I should say the target ID at great depths is the most accurate. I have never dug targets as deep as I have with the SE and Pro coil.
I have seen many warnings about running sensitivity too high on the Explorer. Many people I know run sens between 25-27 and will not go higher. You may want to do some more reading about the sensitivity issue and think about what the result is you're going for. More sensitivity does not equal maximum depth. If you are maxed out and sweeping too fast, I can see missing a lot of targets. Also if the setting "deep" and "fast" are on, signals can be truncated and easily missed. If you are running with the "deep" setting, I think a slow sweep speed is necessary.
What type of hunting are you doing makes a difference too. If you're hunting CW relics, beaches, woods, home sites or hunted out parks you will get different recommendations for settings.