Lawrenzo........ yes, mineralization will 'shut-down' proper/correct ID on SMF detectors.,.,.,.,.,., especially at depth. Some SMF's will be worse/better than others; yet, still within similar ball-park.
In my nearly mineral-free Florida dirt........ the EQX-900 will indeed acquire (just barely) a 14" deep dime.....with reasonable ID...........IF, IF, IF EMI is mitigated. EMI is (by far) my worst inhibitor for BOTH depth & ID.
Yes.........audio gain will merely amplify (increased volume) the existing signal.