Noise cancel 12 inches off the ground, then noise cancel with the coil on the ground. This will tell you the channel it likes for the EMI, then for the ground. Hopefully they agree, if not, use the one it liked with the coil on the ground. Just note both of them. Then try each channel, and note which one gives you the best auto rec/hits the deepest targets. Use the one that works the best. I've found that in difficult conditions, playing with the channel alot, even as you move around to different parts of the site, can be helpful. No guarantees, of course, but something to try.
And yes, it does sound like TTF makes sense at that site, tho the E-Trac will still see silver in iron if trash density is set to High (this is not the default, so check it).
I don't think burying coins and testing them is valid, especially with Minelab machines. I think it works differently on coins that have been long buried (or so I've heard, anyway). This is a test to do sometime, tho, that could all be myth. Sometimes if I find a deep, iffy target that I'm pretty sure is a silver, I leave it in the ground for a while and use that as my test garden for that site (works for testing the best channel for the dirt at that site, and much better than a recent test garden or a test garden at a different site), but it sounds like you haven't gotten that far.
But, definitely play with TTF and channels. At a difficult site, you just have to play with the machine much more. And, I have one site I just can't hunt, its right under powerlines, and the deepest coin I've ever gotten is a silver quarter at 4 inches. I've tried everything, and I know there are deeper coins there. I've even tried hunting in pinpoint mode -- that actually works, until it doesn't -- you can't just keep hunting in PP mode, you have to switch out and back, and its a pain (I could do it at this site, as there were not many targets at all -- if your site is iron-infested, this obviously won't work).
Well, a bit wordy, but HTH.