@SergeiNY I don't have any fringe depth nails or small iron in my test garden. I have nails at 4 & 6 inches and big iron at 12 & 18 inches to simulate my typical relic hunting sites. For what I have, the IR setting where the tone begins or completely goes to the lower tone is: 4" 10d nail @2 or 3, two 6" 12d nails @ 3, a 8" deep 2"X3" piece of flat stove cast iron @3-4, a 12" deep complete horse shoe @ 3-4, and a 18" deep large piece of flat cast iron stove @ 4.
In addition to the actual "fringe depth" coins I tested in the video above, a 8" deep Nickle will low tone @3-4 IR but a 8" deep Copper Cent won't low tone until IR is set to max 5. The conductivity of the target seems to be a meaningful factor in the IR response. This is also shown on the 18" deep large flat iron as it tends to consistently give a false high tone in normal motion tone ID modes.
As far as the effect of dt on unmasking in iron, higher levels(5-6) distort the audio including the high tone. It does the same on deeper isolated targets too. Examples of both of these is shown in a prior video I did on the Legend dt in Minerals & Iron. The benefit I am seeing from dt is mainly in working in mineralized ground. It does help a small but noticeable amount on unmasking in iron trash if one can get past the audio distortion.