So, there seems to be two ways of doing the 4CM Method and I suggest you to try which one works for you and your amp:
”Official 4CM Method”:
QC FX Send 1 → Amp input
QC FX Return 1 → Amp FX Send
QC Out3 → Amp FX Return
This is the one, that you can find in the manual. For my experience this connection is hotter (more gain) and the feel of the amp is different, when playing.
”The other 4CM Method”:
QC FX Send 1 → Amp input
QC FX Return 1 → Amp FX Send
QC FX Send 2 → Amp FX Return
Note: Use SEND 2 as Output on the Preset Grid. This connection method works better for me. Amp has the same gain as without the QC and the feel of the amp is right.
I didn’t change any of the Output(s) gain settings or input impedance. All of the I/O settings have been left default.
Also this INFO is good to know:
Output level will change depending on which cable are you using:
XLR Outputs 1-2:* +9.5dBu*
TRS Outputs 3-4:* +15.5dBu balanced, +9.5dBu unbalanced*
So, more output level is expected if you connect TRS cables to outputs 3 and 4.