I feel each one had some difference in them, not a lot though and those that get to know them well will probably notice a difference.
The XS had a 8 inch coinsearch coil which is one of the best coils Minelab ever made, but it is not waterproof and had the plug in battery. Great performance
The XS2 came with the drop in battery pack and you had a choice of a 8 or 10 inch coil that were waterproof, but these coils were not as nice as the 8 inch coinsearch, Performance was great too like the XS.
The XS2a they added a switch so you can have single tone if you wanted and it too had the drop in battery pack. Of the 3 I tried i didn't fell the performance was as good as the XS or XS2.
The Elite came out with a new battery system which was superior to any of the early one, came out with the new 10 inch Tornado coil which was lighter and really a great coil, they had a 8 inch Tornado too that could be got as a accessory. Minelab changed the controls around a bit so we that had used the earlier one had a problem turning the right control. To me the signals sounded deeper than they actually were and I never found as deep as the XS or XS2 would get.
Now the GT added a few more switches so you could have silent search if you wanted, could turn the iron mask on or off where the earlier one were always on. Also had all metal mode for the relic hunters and prospectors so you could track the ground and lock it if you wanted. I find the GT is more sensitive then the others and it seems to false more on iron, but once you get used to it it seems like you can tell the difference in most cases. I feel the GT is the best of the Sovereigns for those that know it well, but I feel too that those that are new to a Sovereign any one will probably do great for them.
One thing i am seeing is the XS2 and the XS2a and even the XS Minelab or Sun Ray don't make rechargeable battery pack for them, but I have seen a site that makes the plug in ones for the XS and not for the drop in ones, so they would have to have a mod done to them to make them a plug in .
Myself i am looking for a good used XS or XS2 for the right price to have as a extra Sovereign as these were my favorites after the GT.
Rick
