Like you, I have preferred Concentric coils more than DD designs most of the time, and I've used a lot of DD coils since those early Compass Yukon series models from '71ish. There are some DD's that do fit in my selection, such as the 'OOR' coils on the FORS CoRe or Racer/Racer 2, and the equally versatile round 5" DD on the FORS Relic or Impact and also the Racer's. Can't ignore the new open-frame 5X9½ on the Impact, and I have added this to my 'want list' for both the CoRe & Relic. Although I do work a larger-size search coil on occasion, it is more of a rare occasion because most hunt sites I visit are terribly choked with building rubble, dense brush, and more trash than the average Hobbyists would ever want to deal with. You know what I mean.
Two-and-a-half months from now it will make three years since I got the FORS CoRe in my hand, and this has been the longest length of time I have ever spend with one or more Double-D equipped detectors making up my Regular-Use Team since about 1973. Back then I used a mix of Compass, Fisher, Garrett, White's, and many brands [size=small](in that early era when there were many other brands)[/size], but have almost always had at least one Compass around that sported a DD coil. I have also owned eight or more of the Minelab BBS and FBS units that came with DD coils, but I had other units that complemented them better ... and used concentric coils.
To tell you the truth, the 7" Concentric on my Impact was pleasing me so much I didn't want to be without it until wintry cold and/or snow arrived, which is usually by mid-November, and I had planned to shoot you an e-mail about now, in mid-to-late October, to see if you'd like to use my 7" C. for a month or two since you're down there in that moderate-to-warmer climate area in the SW. I figured you'd use it to check it out and then get one for yourself for sure, or I'd just pick it up when I make a get-away this winter now that I have a travel/camp trailer.
However, my 7" Concentric was happily mounted to the working end of the Impact I had along with a CoRe and two Relics and a Tesoro, plus two Nokta back-pack detector bags that had ALL of my other Relic coils, ALL of my Impact coils, and new Nokta Pinpointers, new 2.4 Gig headphones, in-the-box, new 'Hornet' headphones, and a lot more that was stolen when thieves broke into my Tahoe in he hospital parking garage where I hurried to see my Mom.
I do still have my over-size DD on a lower rod that was at home, and the open 5X9½ that was on my other Impact that was home, or at Monte Jr's. with a CoRe and Relic for a detector tote project he is working on for me. So, 1st on my shopping list is another 7" Concentric then a 5" DD as my must-have coils for the Impact. Once you get it and work it afield in a few different settings/applications, I think you'll agree it is maybe the best all-purpose coil for the Impact.
Monte