For me being a former Whites guy I've owned or compared them all and the QXT Pro was the deepest on silver and copper coins in my soil. That being said, in my soil even my QXT bottomed out at about 7.5 to 8" on a silver dime. The 6000 Pro XL was very close but a hair behind it in depth for me. I never cared for the XLT due to it's slugish response in trash where as with less software slowing it down the QXT was very fast at seperating targets.
I headed off to own three Explorers over the years and none of them would get any deeper than my QXT *IN MY SOIL* (which tends to be medium to high in minerals) on silver or copper coins. Sure, it was deeper on lower conductivity targets thanks to the extra higher frequencies, but in my soil a low frequency machine like the QXT seemed to punch through the minerals better. It for sure had a more stable ID on coins at depth than I could ever achieve with my Explorers, and I tried setting them up, reading tips, and hunting various sites every which way I could...digging by audio only, etc, and so on. I feel there is something about FBS machines with all those extra high frequencies that reflect off my ground minerals or something and don't give them as good of depth as others have reported in their soils, high mineral or not. Maybe I've just got the perfect blend of minerals that don't like certain frequencies an FBS machine is generating.
Down the road I came to a point where, lacking something larger like a 12x10 coil for the QXT (man I wish they made one for low frequency Whites!), I was forced to look around again for something new that might get better depth in my soil. Owned many of them in the past and there were only a few I didn't try or compare to yet. CZ6a? Nope, plenty of head to head field tests on undug targets and it didn't ID at depth as good as the QXT. Garretts? Please, I've been there. 'Nuff said. Tesoros? Same deal, and I can't stand not having a good VDI along with tone ID. DFX? Again, seen it in action and wasn't impressed. In fact, using the same size coil I'd put my money on a QXT regardless of how well (and hours you spent) tweaking out a DFX. V3? Surf those forums. I'm not seeing any evidence to support a true performance increase, and again it's tweak city for marginal results. MXT or M6? Also seen in action and, while great machines, they are still getting the depth of Whites from the 90's. T2 or F75LTD? Those interest me, as I've heard some good depth reports from even former Minelab owners.
What I was left with, besides those two machines to try, was either an Etrac or a Sovereign GT. Always wanted to try a Sovereign and I couldn't afford an Etrac at the time, so I thought I could buy a used GT, prove to myself that it wasn't any deeper than my prior Explorers, and then sell it off for an Etrac. Well, low and behold it's been over a year now and I'm real happy with my GT. I can honestly say it's the deepest machine I've owned *IN MY SOIL* on silver and copper coins. With the stock 10" Tornado (much better than the 10" Explorer coils to me) I've dug a V-nickle and an Indian at about 11" deep at a spot I gridded the tar out of with my Explorers and could never achieve more than about 7 to 8" of depth with them there. Threw a 12x10 on the GT and it looks like I'm getting even deeper based on some deeper wheats I've plain out missed before.
Point is that having owned, used, and compared most of the machines out there through the years I was always a Whites guy. The audio and VDI was great, and the machines were built user friendly. I've now built a custom shaft for my GT and am running a lipo battery to drop about a pound and a half off the unit so that's no longer an issue. It has good long drawn out telling audio like I liked on some Whites, a great VDI resolution on gold versus tabs and other junk, and I can even chest mount it right out of the box for some water hunting. Regardless of which Minelab you own (Etrac, Explorer, Sovereign), you can rest assured that no Whites can compare depth and performance wise, regardless of how long you figure out a custom program for a specific site. And by the time you do, you'll look around and realize it's dark and all your buddies with Minelabs have hunted it out and went home for the night.
Whites has gone off the deep in with software engineers instead of electrical engineers. You want to impress me? Then make the machine go DEEPER even in rough ground than prior Whites models. Progress is based on that, not how many more worthless or confusing parameters you can set in some computer program. When they figure that out I'll consider a Whites again.