If you're talking 2 box machines, There is a bugaboo, when trying to make something like this "more sensitive", in order to "go deeper" (which is the improvement that you were hoping for, I surmise?). The problem is this: that if the machine becomes more sensitive to go deeper, then it risks being sensitive enough to start having the ability to find items smaller than, say .... mason jar lid sized object. When it comes to machines designed specifically for caches, one of the goals you'd want to have, is the innate ability NOT to see small objects like that (unless you intend to use it for cross-over coin hunting, that is).
So ironically, "more sensitive" is sometimes NOT what you want, when specifically cache hunting. The old 2-box machines, have remained almost the same (ability-wise), since the 1950s, or heck, perhaps even the ones from the '30s or '40s too! That is: they can effortlessly pick up a toaster-sized object down to several feet, even up to 4 or 5 ft, while simply not seeing anything smaller than a can or jar sized item. What more could you want? That's why there was probably more caches found in the OLD days (of the primitive BFO and all-metal TRs) than are EVER being found today, with our super ultra-sensitive machines we have today! Because "by their very nature", those old BFOs were the perfect cache hunters. While they could scarcely get a coin more than an inch or two deep, yet a hubcap or cache or something would indeed be detected to much deeper depths. So it's the same psychology for the 2-box units: More sensitive is not always better. And I fear that trying to get more depth out of them (ie.: making them "more sensitive") risks making them more sensitive to the small things you want to waltz right past.
And another point: There is a common misconception that all caches are, by necessity, deep. If you can get jar or toaster sized item down to multiple feet deep, is plenty deep. What makes everyone think that oldtimers dug to insane depths, when caching their stuff? So long as the top ground was invisible, and assuming they intended to come back for it later, then what the heck difference did 1 or 2 ft. verses 5 or 6 ft. make? There is simply no mandate that caches are necessarily deep.
