Jim, I too as you know, have been getting the same results as you. I wonder the same thing, that maybe all of these "filters" are robbing my depth.
I have noticed that when I use "difficult ground" I get more chopping of the tones and only surface or shallow coins come in clean, so I have stopped using it and the same with the "dense trash" setting. I am guessing that if I let in or accept nails, I have no purpose for the dense trash setting anymore.
As for fast on, seems to me it does not affect my quality of tones at all, however, when I turn it off, I see a rapid loss of signals, very noticeable . I don't see myself turning off fast any time soon.
Haven't really used deep much other than to experiment with it a little, so I can't tell you much on that. On my SE's, I have had good results with deep on, fast off in sites where there was allot of surface clad (1-4 inches deep) above older wheats (at 4-8 inches) but I had to turn my gain to 5 or lower to be effective with it set up that way and I just listened for the faint deepies. Giving the fact that I am not seeing targets with the ETrac below 5 inches, I don't see where I would use the deep setting.
I have had good results with smooth and long audio's, but it is something to slowly get used to, some days, I just don't have the patience for it, others, it is an excellent way to get even better separation in the trash.
I wish I could tell you a solution, heck, I wish some one could tell us a solution. I myself am at a loss and it is not from a lack of trying. If you still have your SE, see if both the ETrac and the SE air test at the same depths. Dick at ML said I should be seeing 6-8 inches air testing on a quarter and mine does half of that unless I turn it on manual sensitivity and crank it past unreasonable normal usage levels. My SE does it in auto sensitivity just fine. From that, I am thinking there is something wrong and he agreed, saying I should send it in to be checked out.
Whenever I turn my machine on manual sensitivity and push it, the falsing of nails to 1-30 to 1-34 gets much worse and my machine actually ID's worse and gets less depth, basically becomes unstable and sloppy. My best results while maintaining stability have been auto +3, fast on, deep off, neutral soil, low trash. And my deepest coin to date has been a 6 inch deep wheatie on edge that only repeated in two directions. That is the only target I have found at 6 inches other than some very deep soda can tops and large iron relics falsing high, like normal.
I think it would be helpful if guys listed their deepest coins found with what settings they were using at the time, maybe there is a correct way, maybe not.
I now have 55+ hours on my machine and can pluck 20-30 targets an hour with it, just nothing deep.