I owned a DFX a couple years ago. It's a great machine if one wants to put in alot of hours learning how to use it. I had to give up on it because it was too much for me. I traded for an MXT and my finds increased by double at least, mostly because I believe it is simpler to use, although it has some great features all its own.
To your question--------The MXT is dyn-o-mite on all categories of jewelry, as the manual says, due to the MXTs 14 khs which is more sensitive to lower conductive targets such as nickles and rings in the white gold and platinum areas. If I'm not in a superduper trashy area I will set my discriminate at 2 1/2 to 3 and watch for the unusual numbers on the ID. Yesterday I found a trinket that read between 34 and 39 on my meter but steadied on 37 most of the time. My depth indicated 4 inches. At about 3 inches I recovered a nice round charm with the circumference of a cigarette butt and the thickness of one sixteenth inch. That is small! After two years with this machine, it still amazes me.
Now, I do believe the DFX can probably duplicate this feat, since the DFX allows you to choose a 15 khs frequency by itself. But I still say setting the DFX just right takes time and patience. With the MXT one or two adjustments, and a hunting you will go. BTW, I use relic mode 95% of the time. I like that high squeak alerting one to a possible good find.
Hope it helps ya.
Glue