although you consider it an "air test" by laying the targets on the ground, having the proper ground balance setting is an important factor. The coil is, afterall, having to analyze the ground matrix under those targets. That is the first thing that comes to mind with those three inch target depths.
Regardless of the coil, you should hit 5 ounces of silver at depths exceeding 10 inches. Another factor could be the noise channel you are operating on. And lastly........I know the stock coil is 9-inch concentric at 7.5 kHz. But which high freq DD coil are we comparing it to? There is the 6-inch DD, elliptical DD and 10.5-inch DD, all at 18.75 kHz. With a bit more information, we may be able to help figure this out for you. HH Randy
P.S. If I hadn't mentioned it before, the procedure I suggest for those somewhat new to the X-Terra, is (briefly) .....turn on the detector, all metal mode, hold coil horizontal at waist level, properly set the NC, lower the sensitivity to single digit, set the threshold to a level that is just barely audible, properly set the GB and then raise the sensitivity back up as high as you can go (when sweeping over clean ground) without unbearable chatter. You can hunt with whatever level of discrimination you want. But I always suggest setting up the detector in the all metal mode. And until you can differentiate the threshold tone from the fluctuating GB tones, increase the sensitivity after you set the proper GB.