I’m sure I haven’t run QC output into an amp input 
QC output was going to the return of the amplifier, that’s exactly my setup (see as of 5:26):
and that’s exactly how described here Quad Cortex with Real Amp
Are you saying that I had to do it differently, meaning setting an output of the whole chain to Send ?
e.g. this is how all presents look like:
Although I call it 4 cable method (which I will tweak later to fully benefit from it) chain doesn’t use it fully, but just an input and output.
But instead I can try this:
Tried, it works, but didn’t notice any sound difference (late night, will try also tomorrow with more gain). As you can see there is a bypass flag on the CAB - I presume it is bypassed.
I just realized a lot of people in community do it as you said:
https://unity.neuraldsp.com/t/quad-cortex-with-real-amp/6507/9
Shall I also try TRS cables ? Does it make any difference ?
Regarding Master volume control, one of the great things about the Engl Fireball 25 is that even with the preamp bypassed, you still retain Master volume control - unlike many Marshalls, where bypassing the preamp removes the master volume entirely and blasts the full 100W straight into you face.