I've got the 5000 and from what I've seen, all the GPXs from the 4000 on up have very similar features, so I'm going to say that what I've found is going to hold true to them as well.
The soil timings that you choose can have a major bearing on how the detector responds to targets in the ground via signal strength and tone. I'm using my machine to relic hunt with and I've yet to really find any good surefire help other than trial and error and guess work.
Minelab is trying to provide info on their website, via their classrooms there. Jonathan Porter and some of the other guys with many man hours on the GPX are writing articles and such over there, and that is a very good place to start. The problem is, is that the GPX machines are gold prospecting machines and most of the people using them are talking about settings for finding more gold. This differs from the way a relic hunter using a gold prospecting machine to find relics, should set the machine up! So that is the major problem there: lack of info for relic/coin hunters to get the most out of this power.
What I'm having difficulty in determining, is how to figure out what soil timings to choose for your given sites. There is a chart that Minelab has came up with, that you can sort of pick the size target you are after and it shows you which timings would be most sensitive to it in varying degrees of mineralization...BUT it still don't help in determining HOW to figure out how bad or good the soil mineralization is!!! So every time I go out to detect a site, I am always wondering in the back of my mind if I have the correct timing selected to get the most out of the machine.
I've dug deep minie balls with my 5000 that didn't even give a low-high tone or vice versa. The deep ones will just give a low tone and wont have a tag end tone to it. You really have to be listening to hear them break the threshold too; and I'm talking 18-20 inch deep bullets. The shallower ones are harder for me to figure out, because if they are shallower than 6 inches or so, they will actually start to overload the circuitry and you will get a broken signal, kind of like it is nulling out over a piece of iron (if you are using the DD coil and iron disc). I've found out that I can raise the coil and determine whether its a bullet or iron, by the iron will continue to be broken and a bullet will start to smoothen out the further the coil gets from it. So that said again, there is no sure fire way of tone ID on it

By doing your inverted target response on the real deepies that don't give a tag end signal, you are missing those.