Dave, I'd like to comment on this, by pointing out something: For starters, I don't have the SE. Thus I'm only familiar with the PP mode on the Exp. II. And as you know, the PP all-metal mode on the Exp. II is wimpy and pathetic (monotone and ratchet feature, blah blah).
But assuming the all-metal PP mode on your SE goes deeper than your disc. mode does, be aware that EVEN THOUGH you are "discriminating while in all metal mode", yet the decision the machine is making, as to whether a target comes up to a certain tid bounce or flicker or whatever, is no deeper, than if you'd been in disc. mode. In other words: the two modes are over-laid on top of each other. And if, for example, you can get a dime to 10" deep in disc, mode (ie.; the depth to which you get a certain discernable ID or conductive indication), then you are STILL getting the dime at only 10" deep when in all-metal mixed mode. YES you will hear it perhaps down to 11" deep (the deeper level to which your all-metal mode goes deeper), but that last inch of depth (the 10" vs the 11") you are only going to be hearing it in the all-metal tone indication, not the disc. TID tone.
This has been true for all times, for all machines, that offer this mixed mode option. And all the time since the beginning, people have made the mistake of thinking that they are achieving all metal depths WHILE discriminating. It's simply not true. Yes you are attaining all metal depths, but the trigger point of wherver your disc. could reach, is simply laid over the the top of that all-metal sound. Am I making sense?
The reason it *seems* like you/we are getting deeper in the mixed modes, is because of the subconscious reaction of tending to be drawn to double check those deeper whispers. And then sure, once you are drawn to double check things (ie.: center your coil better over them, cross cross them, play with swing speeds, etc...), then sure, you can often time "bring them in" with a centered signal. And if you had merely been in disc. mode, perhaps you would not have been drawn to double check various signals, because of course, perhaps you simply didn't hear them to begin with. But no, you're not discriminating deeper, when going to the mixed mode options of any machine.