You always want to run the MAX at it's maximum potential, ie the highest sensitivity but still stable, ground balanced, using the mode for maximum depth, yet not getting to the point of burn-out from too many signals.
So, for water hunting in freshwater lakes, I hunt in true all metal mode, with the iron audio set at about 35. True all metal mode audio is always the same pitch, no matter ifnthe coil goes over silver or gold. I then hunt using my VDI numbers as best as possible, and also utilize the iron audio to rule out iron. This works best 20-30 feet away from shore. generally, I dig it all in the water, unless I m close to shore. Hunting in the shallow water, near the cut, I switch from true all metal mode to zero discriminate mode, discriminating out iron iron. I don't use the iron audio close to shore, as it is jut to noisy with all the iron junk. I listen for those high tones for coins and silver. I also monitor my VDI numbers in the 50's, as that's where some gold shows up. Pull tabs give a loud double beep at 53, so I just leave them be. Use a large coil for water hunting. I prefer concentric coils over DD's, but that is a matter of personal preference.
Hunting streams and rives is a blast! I have pulled out large cents, rings, musket balls, belt buckles, fishing tackle (of the vintage type) and much much more. Use a smaller coil.....5 x 8 is a great coil! Depending on how much junk there is, you can hunt in either true all metal mode, or zero mode discriminate. I set my iron audio setting to about 35, iron audio turned on, and check each signal VDI number/high tone. Creeks can usually be hunted in true all metal mode, so using the mode will give you the most depth and alert you to even the smallest and/or deepest target. Using a small coil also allows you to guesstimate the size if a target, by swinging it over a target in different directions and find out where the audio stops.
I never use the true all metal mode hunting in parks or sports fields, as I prefer to not listen to every target under the coil. Some discrimination is necessary to maintain some sanity. Hunt in zero discriminate mode, iron audio turned on and set at about 35, hunt and listen for high tones and other tones with stable audio when swinging the coil in different directions.
Relic hunting.....hunt in true all metal mode, as you will probably dig all, and a larger coil is better.
Here's a link to some videos made by Steve Moore from Garrett to help with the basic operation and useable options with the MAX
Good luck!If you hve any specific questions, please ask!