I use my Sovereigns with the iron mask on all the time, it seems to hit on coin close to iron better. In the owners manual it will state that with it on it may false on iron a little more, but will see coins in with iron a bit better, with it off it will null on the iron more and not see the coins that are close to iron.
The Sovereign all hate iron even at the lowest disc settings, but some will bleed though and give you a signals and with some experience you can tell the difference. Now on the GT it has a switch for all metal and when activated the toggle track/lock/pinpoint is now active and depends on where it is set as to what it will do, in disc it is not active and does no good, only when in all metal will it be active. Now I always leave mine in pinpoint as this is the only reason I will use all metal as all my hunting is in disc. Now the all metal is used by some and for prospecting is where the track and lock come in plus a few do hunt in all metal too so it does have a purpose.
Like the F-75 or the Explorers you have used it does take time to get used to the Sovereign, so it will be a little different, but have some patience and keep it simple to begin with and learn all you can about the Sovereign before making it too complicated. To start with I would run a little disc, no notch, threshold on, disc set with iron mask on, sensitivity auto and leave the track/lock/pinpoint in pinpoint so when you switch to all metal it will be in the pinpoint mode. Go slow and listen for the tones. Maybe it will help if you go to a newer area where coins maybe new, but not that deep to get the idea of what is good and bad before going to a tough location where targets are far and few and deep. Like I say make it simple as it will be easier to understand.
Good luck