I have my reasons for not liking the DFX or dual field. They remind me of the XLT and I found it and the DFX had less depth on coins than the Whites QXT Pro. From what I've seen the DFX do in the field I'm fairly confident you can probably beat it in depth if you set up an MXT/M6 the right way as well, or at least with the right size coil. I know for a fact it didn't seem to have as good of depth on silver/copper coins than my QXT, and with the bigger coil options for the M6/MXT you can push them deeper if you had to to beat it even if they aren't doing it with the stock coil (which they might).
But then again, like the XLT, the DFX has so many adjustments that I'm sure with proper tweaking it can get down there pretty good in certain places. Personaly I feel that if I'm going to spend the kind of money it takes for a DFX I'd rather opt for a Minelab Sovereign GT, Explorer, or Etrac. All three of these are pretty even in depth and there is no doubt about the depth they have with far less things to have to tweak in order to achieve it.
Some of the things I've heard about the M6 have convinced me that it's the only Whites currently being made I plan to buy. The MXT would be the other but it's not got the features for my style of hunting (coins/rings) that the M6 has that I feel are better for me, Such as the higher number of tones. Either one of these machines with the right coil can get to some very impressive depths from what I've read. In fact, I think with the right larger coil they might just beat a Minelab with it's stock coil on coins/rings. That would be only in certain types of grounds though, because any of these Minelabs will get down there further in mineralized ground than other machines on the market.
Bottom line for me is that I'm fairly confident that the M6 or MXT would make a great machine to sit along my Sovereign GT, and I feel confident in passing up the XLT and DFX to make that purchase. Can the DFX/XLT get to some amazing depth? Yes, in certain situations and if they the adjustments are set properly, but it's far more work and far more specific to a given site than any of the top three Minelabs. I don't want to rely on the perfect adjustment of every little control and the planets lining up just right to always do that. With something like the M6 or MXT and the right coil selection I'll bet money you'll be keeping up with the big boys (Minelabs) in most situations, at least if the Minelabs are only running a stock coil and the ground isn't really bad.
That's my opinion and I'm sure some high end Whites users will disagree. It's based on my experience with these machines but I've only read about the M6/MXT depths with various coils thus far and done a little air testing with a friend's M6, so that may change when I buy one for myself or have more time on his in the field. We plan to do some GT/M6 depth comparisons as we find undug targets.
It'll be interesting if the M6 with his V Rated 10"DD coil can keep up with the 10" Tornado on my GT. I'm willing to bet it can in some locations and might even surprise me more than that. Even if it can't I'd sure like to see what the M6 does with one of the larger Excelerator coils such as the 10x12 against it. In that case it might have the upper hand in some ground conditions, but then here's me picking one of those up for my GT to beat him again if that happens.
Another thing I'm very curious about is these two machines going head to head on gold rings, since they both have a reputation as being king of the hill in that respect. Since the M6 is based on a good higher frequency to hit harder on gold, but the GT also has multifrequencies in that range, I'd expect that the M6 might hold it's own on rings better than it would say on coins against it (which prefer a lower frequency like the lower ones the GT also has available). Again, I'm sure the M6 will hold it's own in certain ground conditions but might have trouble with others.
When we get around to doing these comparisons we'll throw up some field testing info...