How do you control volume for a specific instrement in a sub group being used as a moniter mix with out afecting the main mix

This unit will require an amp. I have set up several mixers and I
do not watch output level numbers, you can get desired volume using amp. It does no good to drive input levels high output levels high and the turn down amp. Lower your inputs so no light flashes set master gain to -6 output and then raise amp volume. I have set up 16 mic mixer set individual levels and the feed to main mixer and controlled levels with one fader. I have taken mixes from main mixer to another mixer to control audio on video feed. A good band does not let member's set their levels . the audio person set levels, bad bands have members to set own level with resultant a mess that gets louder and produces sub par event. Remember good bands sound good and bad bands only sound louder, and bad

Simplest is to use the LR button at the bottom of the channel strips you want to send to the main mix. Make sure you set the gain for each channel in use so that the clip light just occasionally blinked then back off a fraction.
If you want to use sub channels assign the strip to the appropriate one 1/2 or 3/4 and the assign the sub to the main mix and bring the subs up.

send a signal thru the sub 4 insert and see if it comes out the sub 4 output. You may have jarred a ribbon cable loose in transport.

The AUX output is your best bet as it is independent of the main mix, so the main volume control won't affect the recording. You will need to turn up the AUX control for each channel you wish to record, and also make sure that the AUX master is up. Depending on the software/ hardware you are using to record, you should be able to set the main record level using the AUX master. Start by setting all the AUX controls to 12 o'clock and adjust from there.

From page 84 of your user manual, the signal flow diagram shows that the two effects busses can be sent to the sub busses. You CANNOT send the subs through the effects unit since that would loop... so the effects that are selected from the channels onto the two FX busses which are then processed into the effects signals, then can be ADDED into the sub group signals. Make sure the subs are NOT selected to the mains and the solo of the subs is not set Nor is the sub mutes. You say the subs are showing signal... MAKE SURE THIS IS with the indicators set to display "Output" as that is the one that counts.. If the output when the sub is selected shows no signal, then likely the mute for the sub has gotten set.

Possibly you are using the audio from the subs or the Aux which are NOT controlled by the slide faders?

Sometimes users turn up the stage monitors to the point they can't control enough with the mains.

Make sure your system passes the audio to the amps from the XLR plugs for the mains on the back AND you use BALANCED lines to do so.

The MON faders ONLY control the output to the MON L and MON R jacks, NOT the split use of MAIN/MON outputs. You MIGHT try varying the MONO fader and see if it varies the MON part of the split output.
Best to use the unit as MAIN L and R to main speakers and use a stereo amp or POWERED monitors driven from the MON L and R jacks. With this, the MON faders will control your monitors as you would like them to.
ALSO make sure ALL equipment connected together is powered from teh same recepacle power system. Powering interconnected equipment from receptacles at opposite ends of the room is asking for damage due to ground bounce and faults.
PLEASE read my tip on here about a HAZARD of speaker polarity if you use Speakon to 1/4 inch cables.

The main mix controls ALL outputs simultaneously.

The mic channels are controlled individually by the sliders.

It sounds like you are on the verge of feedback where the mics are picking up the PA speakers.

Make sure you have loaded the support SW and make sure you have disabled ECHO as the mics may be looping back through the computer causing the rattle.

Go to www.korgpa.com/pa_root/pdf/pa800/Pa800-200UM-ENG.pdf.Page 226 has the audio output section.Page 19 has the rear panel explanation. Left/Right: These are the main stereo outputs. Use them to send the final stereo mix to an external device. Connect either to get a mono output. Set the output level with the Master Volume slider. 1/1: These are the sub outputs. Use these to create a stereo submix of just some tracks or to output just a single instrument to be mixed alone or to be processed or amplified externally. Not: the Master Volume slider has no effect on these outputs. Signal is sent dry with no effects applied. Hope this helps.

Are you sending the main mix to the monitors or are you giving them their own mix? What about using the sub groups, use 1 for your monitor send.