I had a breakthrough in my Mark III high gain tone journey a couple weeks ago & here it is. Before this I've always gone straight to the lead channel when I wanted this tone, and used the standard settings on the knobs & GEQ you'd expect. Moving on from this I started trying some higher than normal presence settings to take the blanket off, and pulling back on the 6600 slider to compensate.
https://youtu.be/n-rqJAUmkMI
This was good, and opened some things up for me. It still wasn't exactly what I was looking for tonally in rhythms, though. It's still a little compressed & fizzy there. So, I kept tweaking, and kept ignoring how everyone else says they run theirs. I actually switched over to the Rhythm 2 channel & slammed it with a 20db boost, and knobbed up these settings while running on my bridge EMG 81-
Volume 10 pushed
Treble 10 pulled
Bass 1 pull or push to taste, you can kind of compensate either way with the 80 slider
Middle 4 pulled
Master >1.75 pulled
Lead pushed (off)
Lead master pushed
GEQ enabled & set to taste
Presence 10
Simulclass / Class A to taste
On your guitar- volume at 10
On your guitar- tone at 0 (This was the big fizz killer for me- dump the tone & compensate with 6600. Also stops the string wipe noises.)
Input boost at full tilt
To me, this gives the same basic gain & saturation as running the lead channel on more traditional settings with the gain around 7 and no boost, but it really opened up the note separation when chording & I think gives it this killer tonal edge. It also drops the bass flub just a little compared to the lead channel with the same settings. It may change next month but right now I'm super happy with this sound for playing heavy. I can't wait till I get it in a room where I can turn it up past 2.
Side note, I also switched the Mark III from my Mesa recto V30 cab to a Marshall 1960B G12T-75 cab, and I think it really suits it better. It was another step in taming the fizz with the tall presence settings. Marks take a lot of work to dial in but when you do they kill!
https://youtu.be/NdARuM_4QJ8