I ran 2 NEL coils on my AT Pro, the NEL Hunter and the NEL Thunder. Both are DD coils.
Right off the bat, I knew the NEL coils were winners. To begin with, while the NEL Hunter is actually slightly larger than the stock AT Pro coil, it actually had better target separation. I hunted an area that was riddled with iron rebar under the sod, and never could get anything other than iron signals off of it with the stock AT Pro coil. The first time I hit it with the NEL Hunter, I pulled a gold ring off, at 6" literally settled onto the rebar within 1/2 inch. I was tickled pink.
I purchased the NEL Hunter with the purpose of adding coverage to my swing, and instantly my find rate went up accordingly. About 9 months later, I went to the NEL Thunder with the goal of adding more coverage again. I've been very pleased with both of the coils. Their ability to separate is slightly better than the stock coil, but as with all DD coils, if the targets are exactly in line with each other (like 2 along the line of the DD cross over), it'll sound once. Hitting targets from multiple angles is a must.
Personally, I know folks who love smaller coils to get between the trash, I simply swing over it a couple of times and listen to all the sounds, and dig the ones that I like. Never had any problem pulling gold out of trashy sites with the NEL coils.
When I switched to the AT MAX earlier this year, I tried the Hunter first (worked fine), and then immediately swapped out to the NEL Thunder. The thunder wasn't supposed to be compatible with the AT MAX (NEL apparently makes one specifically for the MAX), but I've experienced ZERO problems with it. Everything from Florida beaches to school and parks... it's marvelous.
One thing to note, though, is that the bigger the coil, the heavier the lever. Your arm will take some getting used to the extra weight. For me it took about 2 months, each upgrade, to get back to swinging 4-5 hours at a time, dropping all the way down to about 2.5 each upgrade, with sore arms after.
Cheers!
Skippy