I have a T2 Classic and concentrate on relic hunting. I'll usually keep my disc at around 10-20 and dig everything. Even though I pick up the iron, Colonial , Rev War and Civil War era sites can yield some pretty cool finds. One of my favorite areas to hunt I did dial it up above 40. It was an old railroad road bed and I was just worn out from digging old railroad spikes all day!
Because the areas I detect have a pretty good variety of soil types I have to adjust my Sens up or down some depending on the area. I usually can't go above 90 without getting some false targets, especially in our red clay areas. Usually I keep it at around 80-85. I'd strongly recommend rechecking your ground balance if you you notice you're digging in a different soil type then where you originally set it. I average 40-50 range on GB in most areas but have run into some places that it finally balanced as high as low 70's. I also run 2+ tones.
I'll freely admit I haven't been running any all metal modes yet in the field due to my inexperience with the machine. I'm still getting that side of it figured out, playing around with it around the yard. If anyone responds to your request with tips/info on that mode..I'm all ears too!