I'm greatly looking forward to your tests and reports on all the above. I *love* this sort of testing. Just make sure there are no iron (threshold will null) or any other items in the ground for a good distance around where you plan to stick the coin in the ground to do the tests. The way I do that is to first scan the spot in auto and then at various settings of the sensitivity. It's a little tricky but you'll find that the spot might look like it has iron in the ground only because you've now raised sensitivity too high and thus it's nulling out from that. The trick to deciding which is which is by slowly moving the coil around over the same area. If it doesn't consistently null then you can bet there is no iron and that it's just sensitivity being too high. After all that then stick the coin down in the ground. A good starting point is like 7". If you can hear it fine over a wide range on the dial then stick it another 2" down. You'll reach a point where the only good ID (or at least the easiest one to obtain with less work) will be a very specific spot on the dial. I sometimes take a good ten minutes playing with sensitivity and all that to calibrate it because once I find that sweet spot I'll re-do the entire range of sensitivity all over again, going to high and too low, until I confirm at least 3 times that the same spot on the dial is producing the best or at least easiest ID. Also, don't charge your position over the target when doing this. Often the coin will be easier to ID swept from one direction versus at another angle. Keep your feet in the same spot. Best way is to move the dial a bit then do short/fast wiggles over the coin with the coil just barely skimming the top of the ground as you'd normal hunt. You don't want to scrub hard if that's not the way you normally search. You want to have the coil at the height you normally hunt at, which for most people is just kissing the top of the grass blades.
Once you do all that jazz above and find the "perfect" spot for the sensitivity then do some normal wider sweeping over the coin. Increase and decrease your sweep speed and notice which speed seems to hit hardest/best on the target while in normal "search mode" like this. Try to use that speed when hunting now because as some of us have found sweep speed for best depth is tied somewhat directly to the sensitivity setting. The lower the setting the faster the sweep needs to be, and the higher the setting the slower. What is interesting to note though is that I'm calibrating by doing the short/fast wiggle. Most people believe that gives the best ID on a deep target, and as a result I would guess that's also why my sensitivity setting ends up being pretty darn low. If you were say calibrate sensitivity on the dime by doing a normal and slow sweep speed then perhaps you'd end up with a higher sensitivity setting. However, I don't think that will give the same depth since the "wiggle" has always been said to give deepest/best ID. What I'm saying is that with that calibration method it ends up being that what I would call a medium sweep speed in search mode seems to give the best hit while searching for me. Once a target is hit on, though, wiggle short fast over it. Don't assume it's junk because your normal search sweep gives a junky response or something lower in a VDI # on the scale. You'll be surprised to find that the wiggle can take a junk or "tab" like signal and turn it into a perfect coin signal. That's why I keep saying to listen for any reaction of the threshold, be it a null, junk response, or whatever...and wiggle fast/short over that target to see if it's going to morph into a good signal you'd want to dig.
I'd also like to see you do some head to head comparisons of that Etrac and Sovereign using various coils. You said "I believe that the Sovereign is as good and maybe better in some cases (than the Etrac)". I also believe this, that the GT can get deeper and give cleaner ID's than the Explorer or Etrac if conditions are right. I've also said the reasoning for this in theory many times before- I've heard some people say that the Sovereign has gone deeper and gave more stable ID's for them at certain sites compared to their FBS machines. That could be due to the more/higher frequencies (beyond what's useful IMHO) of FBS machines reflecting off ground minerals or picking up more RF noise. I even remember a guy talking about how he's never seen larger coils (like 12" or bigger) run as smoother, deep, or as good on any machine he's ever owned compared to his Sovereign. Some beach hunters also claim FBS machines just don't work as well as their BBS Sovereign at certain beaches. Even if you don't want to believe any of this it can come down to the VDI on the Sovereign being tied more directly to it's audio output than the FBS machines, resulting in more stable ID at depth. As I've also said more than a few times, I've never seen a machine with this stable of ID, and that's real surprising considering how how of resolution it has. Machines with much less detail in VDI that I've owned don't even seem as rock solid on a target. If it's a good target (coin, ring, button, etc) the ID *will* lock on and not float all over the place on you. That's a powerful tool in deciding what to dig or not to dig, and you don't even have to consider just what the number is. It can be say 136....Not a typical coin signal, but if it keeps that ID and sounds good from any direction you had better dig it. The Sovereign is telling you it's "dig worthy" and more often than not it'll turn out to be something good. An old button, token, cuff link, gold ring, or some other trinket that most would consider to be a keeper. I'm continually amazed at just how well the Sovereign "tells" you by it's ID stability and audio just what you should go after. It's hard to explain but as I'm fond of saying this is also one of it's features that I feel is better than any other machine I've owned.
Just remember to play with your sweep speed and calibrate by using the wiggle, then calibrate by doing a normal sweep at different speeds to see what gives. One might be much deeper than the other, but my money based on my experience is that calibration via the fast/short wiggle and then using a somewhat medium sweep speed gives the best hit on a target.
200
I think Kellyco can sell the same Coil for these American 
