Tom is right; I would try, if you can, an E-Trac or SE Pro and see what your results are.
HAVING SAID THAT...a 12" coin is a DEEEEEEP coin for a detector, any detector. I am surprised in your tough ground that your other machines hit this target so hard (though high frequency machines deal better with hot ground, so that might be why the AT Pro does well; still, FBS should stand out in bad ground, also). In my experience, a 10-12" coin with an Explorer is a very deep coin...getting near the fringe of detection depth. I'm at a loss as to why your XS won't hit the coins as well as your other units, in your hot ground.
One thing I will say is, "scrubbing the ground" is THE way to hunt an Explorer. They for some reason are KNOWN to "not like" an "air gap."
And, you are right, sensitivity TOO high can result in a LOSS of depth, especially in some mineralized ground. So, you have already figured that out -- that for your soil, 14-16 works best. GREAT JOB on that; that's the value of having coins buried in the type of ground you hunt in; it allows you to tune your machine most appropriately for your ground.
One other thing I will say, and that is that DEPTH is not the ONLY reason the Explorers do such a good job on silver; I might say it's not even the MAIN reason. There is just something about the way these things hit on silver, in the tone, and in the ID, and in the fact that there are fewer targets for the FBS machines that "mimic" silver than on a single freq. unit, such that once you begin to recognize how silver "sounds" and "acts" with your machine, you will find that you can really, really hone in on silver coins in a way that, at least for me, I have been unable to with any other unit I've tried. If you gave me an Explorer that sees a coin at 12" max, and another unit that sees the same coin at 13" max, I'd STILL take the Explorer; they are THAT GOOD -- if your hunting goals revolve around finding silver. If they DON'T; if you hunt for MANY things, including gold, and relics, and such, then an Explorer may not necessarily be THE best unit for those things (capable, for sure, but maybe not "top dog.") But, for SILVER specifically? Explorer or E-Trac is where it's at, IMO and in my experience. Period.
Finally, two things -- good headphones will help, but I don't think that would be the reason you aren't "hearing" a coin deeper -- especially if you have your gain set to 10. At 10, a deep coin will be as loud as a shallow coin, so I don't think your problem is your headphones (not that there aren't subtleties that you will be able to ascertain with better headphones, but I don't think they are the issue in this case). Second, I really feel that if I am right on the XS having only one "number" (a conductivity number), then an upgrade to an Ex. II or SE/SE Pro might be worth considering...as I find additional, good dig vs. no-dig info in that additional ID info, be it in digital mode, or in "smarfind" mode (the "matrix" screen). If you can get your hands on an Ex. II, SE or SE Pro, or E-Trac to test on your buried coin(s), see what you think...even if they are just a "bit" better on that deep coin, the CO/FE information on those units would be, I think, a really nice upgrade over the XS.
Just some thoughts.
Take care,
Steve