"Gain: LOW
Power: OFF
Low Cut: OFF
Level Ctrl: OFF"
Those will be the most appropriate settings for most all handheld recorders using internal microphones to record live music performances, regardless of venue.
Gain depends somewhat. Low is likely the best choice if the performance is amplified. Mid or High could be appropriate if the SPLs are lower without strong dynamic peaks and remains that way throughout the entire performance, such that if gain were set to Low the meters would otherwise only peak significantly below something like, say -20 dBFS at the loudest parts. If there is any chance of clipping / peaking at 0 dBFS or above on the meters, set gain lower. Checking during the opener is good practice, but leave some additional headroom for the headline act as that will almost always be louder.
Power refers to the provision of Plug-In-Power to input jack for external microphones. If using the internal microphones built into the recorder this setting is not important, but leaving it off is best practice. If you were plugging external microphones that require low voltage powering straight into the recorder, without external powering from a battery-box or preamp, this would need to be switched On.
Low Cut should generally be switched Off for music recording. If employed, the low frequencies will be attenuated which typically makes the recording sound thin, less natural and less engaging. EQ applied afterward is a far superior way to reduce excessive bass or address other problems in the low frequency region. In that way you can listen while determining where, what and how much to cut, and try various setting and tweak them until it sounds best. Low cut is mostly intended to reduce wind-noise and rumble, and can be useful for other types of recording such as dialog recording, where a wide frequency response and good extended low frequency response is not important to the end result, but may be useful as a last resort when recording something very, very loud with so much low bass that the recorder can't otherwise handle it without distorting.
Level control should be Off for all music recordings. Set recording levels manually. Otherwise the recording will suffer from automatic volume changes whenever anything gets louder or softer.
Other stuff:
As RyanJ mentioned, arrange the recorder so that the microphones are pointing toward the stage and have a direct line of sight to both PA stacks. Get the microphones up as high as possible/practical.
If there is wind, use a foam windscreen or some kind of audibly transparent fabric over the microphones. If no wind down there in the quarry pit amphitheater you can ignore this. To be effective it has to fully cover the microphones without any "air-leak" openings near the mics. Makeshift fabric windscreens are generally most effective when the fabric is thicker (a sweater, fleece, faux fur) yet must also be as audibly transparent as possible, and those two attributes tend to be contradictory. Check potential windscreen material beforehand by holding it up to your ear and listening for how much high frequency content makes it through, as it is the high frequencies that will be most attenuated by the material. Make some test recordings with a fan blowing to get a idea of effectiveness and high frequency attenuation. Fixing a dull sounding recording with EQ afterward is far easier and more effective than dealing with low frequency wind rumble on a recording.
Very light rain should not be a problem. But bring a ziplock bag that he recorder will fit in just in case the rain gets heavy. The plastic bag will not block the sound to any appreciable degree, the biggest issue is potential for the bag to make rustling noise when handled or by raindrops impacting it directly, but if the rain gets heavy, the recording is going to suffer regardless.
Check and format the SD card in the recorder beforehand. Check your batteries. Bringing a spare SD card and batteries is always a good idea.