705 coils

They could. I don't hunt with my Concentric coils at all anymore.

Often the signals that you chase or disappear are the edges of junk targets that are caught by the edge of the detection field. Then upon giving the machine a better look at it, it determines what it really is.
Focus on repeatable clean tones that you can isolate.

Longhair, what coil would you recommend for general all around detecting for your 705? Ive been using my 18KHz 10 x 5 DD coil a lot lately with good results, but the pinpointing stinks. (one of the reasons Im using it is to get better at it). I like its performance as it really brings out those lower conductive metals, but at the same time it sings well for the higher conductive ones. The concentric coil does a lot better at pinpointing, but something seems to be missing sometimes with the target detection; similar to what dirtfishin wrote. For now Ill keep using the DD to get better acquainted with it. If you just had my 2 coils, which would you favor?

Thanks for your help.

That's a tough question to answer.
You probably need a big one and a small one. Beyond that, frequency would be dictated by conditions and targets sought.

I personally think that the 10.5" MF DD is one that every Xterra owner should have. It's the most honest, and typically gets good depth. And if you're going to hunt trashy sites you'll need a 6" DD. Being primarily a coin shooter, I like the 3kHz Digger, but if I were ever to want to go prospecting I'd have to have the 6" HF DD.
You already have one HF DD, so that frequency is covered unless prospecting is going to be a more important role, in which case I'd sell the 5x10" in favor of the 6" (for it's sensitivity to sub-gram gold). The 5x10" covers ground like a 10", but is depth limited by it's width. A 10.5" DD would cover as well and see deeper. But if trash density is high, then neither 10" coils would be suitable. A 6" is the way to sort through trash, and in a DD you only have two choices, HF or LF.