Here's the complete summary of what I've posted on this forum many times:
Lots of bands have been running their guitars in stereo for the last 20+ years in a 5,000-seat theater or smaller. Unless you are talking about an arena or a stadium, running guitars in stereo is actually desirable, as it offers benefits such as "non-clashing" with other signals that are mixed in. Stereo sound is easier in terms of mixing, as you can SLIGHTLY pan instruments left/right, so they would not all be clashing together. I'm not making this up, several pro sound guys told me they prefer stereo guitar signals. If you are playing at a stadium, very few mixing engineers will run a stereo mix. Maybe just a few times during an entire show as an exception to the rule, for some kind of a special effect. Stadiums are difficult with the sound distribution to begin with, the last thing you need to worry about is stereo mixes. And there is no easy way to decide, which side is left, which side is right, otherwise you will deprive half of the stadium of sound when you do this, and the mix will only sound good if you are sitting dead in the middle. The distance from the left and the right sides is just too great in a stadium, compared to a theater.
What exactly do I do in terms of stereo mix in my presets? I mostly just have ping-pong stereo delay in the majority of my core sound. A few songs have very exaggerated left/right delay but only in a few sections. But I dialed it in so that both sides of the concert venue will be getting the same amount of information, and no side would get more musical notes than the other side. For that reason I added my delays in parallel. I play a note, it is heard in the middle, then it is followed by a repeat on the left, then on the right.
Other ways I use the stereo mix is with stereo panning on some little fills/solos, but nothing crazy that would make one side silent, while the other one is blaring. For example, I have one song where when I rock the wah pedal, the sound pans left/right, but not 100% left/right. Another example is one of the songs has a chaotic ending, and I have the tremolo with left/right panning. These are just special effects that I use very rarely compared to my main core sound.
I go: stereo to FOH via 2 XLRs (L/R) and mono for my personal monitor mix via 1/4" L. I need to feel the guitar chugging, so I run my monitor mix into Mooer Baby Bomb 30W power amp connected to a speaker of a combo amp, or my own 12" speaker cab. If the combo amp has the speaker hard-wired, I go into the return of it, if there is a return. Otherwise as the last resort, I just plug into the combo amp on the cleanest setting. I prefer a single 12" speaker, it just has the best focus and clarity for me. I absolutely avoid 4x12 speaker cabs, or Marshall amps. They just make the sound on stage too loud, while simultaneously not being able to hear what the heck I'm playing, and the sound guys always ask me to turn it down... while I cannot hear anything.
My XLRs are at fixed mic level, and the 1/4" is at line level, allowing me to adjust the stage volume loudness with the volume knob. I have it dialed in so that if the volume knob is on around 2, it's my home base. I usually make slight adjustments to have more/less stage volume within the first couple of songs.
I do not remove the amp/cab sim from my personal monitor mix, because otherwise the delays which are post amp/cab would be difficult to set up. I just run the same exact mix to both a physical amp and FOH, as lots of people do. I typically do not use the Global EQ, but sometimes if the venue is small and the on-stage amp interferes with the mix, I may dial out certain frequencies with the guidance of the sound guy. The GE would only be applied to 1/4" mix.
A few times when the stage setup is more amateurish, I went in mono to FOH.
Helix sums your signal to mono automatically, no need for any adapters. In fact, if you have an adapter that adds left + right signals, the signal may become hotter and start to distort. This is something I discovered a few years ago.
On the Helix I have been experiencing a bug where 1/4" Left/Mono doesn't immediately sum both sides, until you plug something into the right side momentarily, then unplug. Some people have experienced the same software bug with the XLRs as well. I have had to do this plugging/unplugging since the beginning of owning the Helix. On 3.7 firmware, that issue seems to have been fixed, but I still verify that everything is summing correctly by using a "Sound Check" preset that has exaggerated ping-pong stereo delay. Before I start playing a show, I always make sure to verify that left + right are summed correctly. I know what it's supposed to sound like, so if I don't hear the familiar delay sound pattern, I know something is wrong.