The older I get, the less I use larger coils. In fact, I seldom use anything larger than the TS800 with my Advantage. The TS800 is nearly as deep as the TS1000 (within 10%) and provides me all the depth I need for coinshooting the fields. And, I can work the smaller coils around corn stubble and weeds better than the larger coils. Granted, the larger coils will cover more ground with each sweep. But with my health situation, it isn't worth the trade off.
With that said, if you don't mind swinging the bigger coils, the deepest coil I have ever used with my Advantage is the CoilTek 15-inch WOT. A few years ago, I spent quite a bit of time looking for a couple post hole banks. (never found them) I was pulling fist sized targets at depths of over 24 inches with the WOT. What surprised me is that while being that deep on the larger objects, it was still sensitive to coin sized targets at "normal" depths. If I still had the WOT I would send it to you to try out. But I sold it last year. I do have a TS1000 if you want to borrow it and try it out. Let me know. HH Randy