First of all, I would like to extend a big thank you to everyone who contributed to this case. Your feedback was extremely helpful in getting everything sorted out (to my frustration, before reaching out to community I have raised a support case with Neural - 5 days later and after several reminders - no feedback at all, I have a real concern about support they offer, especially for newly introduced devices to the market).
A lot was shared here, including some mistakes on my side. To help others who may face a similar situation, I’ll summarize the key takeaways below.
To begin with the initial problem: when running QC into the return of the am’p’s FX loop, I experienced an unnaturally boomy sound. Tried to apply EQ it did not change anything.
That’s mistake 1. I have applied global EQ, but did not apply it to any of the outputs. Must be done like here: (see “Assigned” in green)
My go to EQ setting to eliminate low end frequencies is to set to High Pass filter on the level of 169 Hz. This is a generic configuration which works fine for the majority of presets via power amp of Engl Fireball 25 equipped with two KT77 tubes (similar to EL34s). Layering amp capture or model adds this boomy effect (presumably because a power amp of the model goes on top of power amp in the amplifier).
Short clip with EQ OFF:
and EQ ON:
While troubleshooting I have faced another issue running a preset via headphones, that’s Mistake 2: Global CAB/IR bypass also bypasses headphone out, therefore you can imagine the sound without a CAB, I wrongly assumed that bypass does not include headphones (why would you run a signal to the headphones without a CAB? definitely point for improvement for Neural).
Problem 2. Resonance and vibration in some of the presets. Reason - return of fx loop overload, solution - adjust your INPUT and OUTPUT levels (they might not necessarily be clipping (and never did for me), but might affect how amplifier is handling an input signal).
Then a 4 cable method….Mistake 3. Normal presets with INPUT and OUPUT won’t fly, a specific one with FX loop must be used to properly use the chain. Before implementing it fine tune your QC with a simple path:
Guitar > QC Input 1:
QC Output (3 or 4) > Amp FX return loop:
and nothing in front of an amp:
Then after this setup is fine tuned, it can be reconfigured to a 4 cable method and with proper presets in use. I like using preamp of Engl Fireball 25 with HM2 or Tubescreamer (I tested them next to another: QC capture and real one - it is almost impossible to tell apart which is which even sometimes a real one sounds dull compared to captue). So having an ability to switch back and forth between real amp and preset is nice. Point for improvement for Neural: factory capture must indicate settings, going over Cortex Cloud to dig into description is not always suitable.
Methods described above worked perfectly fine for me, you have to indeed adjust FX levels to avoid massive volume mismatch.
Using Return of FX loop on some of the amplifiers you may lose Master Volume control - so before trying out, bring volume down on QC, this is not the case for Fireball, you retain a control even if preamp swapping is done.
Case is closed with the help of the community (while no feedback from Neural).
Thanks again @xush @Aviraleads @PickinPete @SelfTitledLP !