I will just add, don't be afraid to run the gain lower while you get used to the unit.
LISTEN to the tones. The MMK has a lot of tonal nuances in 3 and 4 tone mode. 4 tone mode takes a bit to get used to. I suggest learning/listening in 2 and 3 tone mode first. You can try GEN and DEEP modes but I would not venture into that area to start learning the unit. BEACH is cool though as I have used it on the wet salt sand here in NJ and it works well.
Make sure you ground balance and do it every so often just to keep the unit running smooth. Also, if switching frequencies to cross check a target, re-ground balance in the new frequency first. You really don't have to, but when doing so, you know the MMK is now set up to run optimal in that new frequency. Sometimes it's fun to hit a target that sounds good in lets say 19kHz, and then switch over to 5 kHz to listen to any differences or to possibly get more target info.
Repeatable good sounding high tones, without any'scratchiness' or 'harsh' type sounds are what you want to listen for while coin hunting. The ID system is good, but I don't rely on that much. I like listening. You will begin to learn in a few hours how the machine reacts to iron and misshaped (ie not round) targets.
Great feature is you can customize the tone on the target breaks, so set it up to what your ears like best.
In my testing as well as live hunts, silver/copper coins do sound the best and the unit gets the best depth on them in 5kHz mode. But as always, ground conditions dictate what to use as well as the type of trash. In heavy iron I have found 19kHz is better as it cuts down on the iron falsing and the unit can determine ferrous vs non-ferrous targets better in that frequency. But aluminium and foil get lit up much more....which if hunting for gold, is a plus.
Sturdy built unit. I like mine a lot.