I have my Quad Cortex connected to a Marshall JCM 800 using the 4-cable method. My idea is to use the drive from the JCM but send that exact same sound to the mixing console and monitors via XLR with a cab simulator.
You would need to split the signal that’s coming into the QC from the JCM 800 to a separate QC lane. That new lane would need to have a cab or IR block + be routed to output 1 or 2. The original lane or signal path that you have would need to then route back to the JCM 800 amp return from the QC.
I will say this isn’t going to give the greatest sound to FOH. It’ll basically sound like a distortion pedal plugged into a cab sim. You’re not using the power amp in the cab sim sound, and that definitely plays a big role in the sound of an amp like a JCM 800. Yes I know some people do that routing, but I don’t think it sounds great just using the preamp to a cab sim.
Routing would be-
Guitar in to JCM 800 input.
JCM 800 send to QC input
QC lane 1 set to Input 1 and Output 3. (Tweak output 3 by making sure the QC output isn’t changing the amp sound or level. Adjust this in the I/O menu.
Splitter from QC lane 2, and the output of that lane is Output 1. This lane has a cab sim block.
The QC has a bunch of Marshall power amp captures that come be very handy in that particular scenario. Insert them between the return path from the amp and the cab sim, and done.