Ah, now I understand more what you mean.
Your CQ also has a line input that doesn’t use a microphone preamp, namely the ST IN.
If you connect your device there in “line” mode and only use the L socket, it will be used in mono.
This way you can try out whether it really makes a difference whether you connect your preamp to ST or the jack input of a combo socket.
When comparing, set the adjustable input so that it is really as loud as the ST IN!
For a second comparison, use the XLR input again and switch your preamp to “instr.”.
I assume that the differences between all 3 variants, if the levels are identical, will be marginal.