@sledge76 Try a buffer in front of the Quad Cortex. That seems to remove the 3000hz whine that I hear.
Here is a silly clip of me turning on and off a buffer, and I hear the whine disappear when the buffer is on
https://m.youtube.com/watch?v=_21W7bBqa9c
You are not the only one, buddy. Speaking on high gain captures and PA/Inear (Desktop monitors are completely different on response), all around 4khz is TRASH. If you make a wide strong cut on 4khz and you on/off it you will hear how much crap are in there. I am still having a battle with this. The part without IR for my poweramp+cab sounds amazing with only a “modest” narrower cut on 4khz. But for PA and specially inear, those high frequencies are a pain in the ass. I tried TONS of IR, using eq after the IR…and still not 100% comfortable using inears. If i rolloff the highs i need to go until 5-6 khz, but then it sounds very dark. I feel the Q for the rolloff is too much radical. The last thing i am trying is using the software cabinetron. It has lots of options and the eq is more tweakable than the eq on the qc. I used the “tone match” function after all the taming i could do using the intro for “Firepower” of Judas Priest as it is only guitar with no “harmful” highs. I will test it later on the rehearsal room. Wish me luck 
4k is a notorious frequency in electric guitars that gets cut slightly even with real amps for mixing
I think different people are talking about different things in here.
Sledge76 I think was talking about the video that I posted: the Quad Cortex has a whine at intervals of 1500Hz even when nothing is playing. It goes away if you use a buffered pedal in front of the QC (as demonstrated in my video). Oddly, the Nano Cortex does NOT have this whine (using the same captures) and is quiet without a buffer.
Other people in here seem to be complaining about the crackling artifacts as notes decay. That’s definitely a thing also with the QC. Not sure it’s as noticeable in the Nano using the same captures, but I’ve not done A/B testing.
And others seem to be complaining about midrange content of the tone while playing. That seems like a totally different issue, probably down to the EQ and miking technique.
You are right, a lot of different things here that point to the biggest weakness of the QC: The noises
Didn’t tried the nano but if with the same capture you don’t have same issues it’s something not normal. I hope it’s not hardware related and at some point the QC can become quieter. 
Sorry for delayed feedback. I tried adding a buffer but it didn’t change. Do you mean an actual HW buffer pedal? I don’t have one, but even if I try, I suspect it wouldn’t change the sound because I tried with/without a guitar plugged in.
If you mean software buffer pedal, I tried adding in a flat EQ block and Green808 overdrive block with zero gain, but those didn’t change the result. Could you tell me the exact buffer block you tried?
Yeah @sledge76. An external pedal with some type of buffer (not true bypass).
@DDguitars in this clip, you can clearly hear a “whine” that disappears when I turn on a buffer pedal placed before the QC. That whine is right at 3000 hz.
Also what’s the power supply that you’re using for the pedals before the QC? Not sure if that was mentioned or the exact chain?
It was the QC power supply. Just a typical cable, maybe 10 feet. I returned the QC and use a Nano (with the same cable) because the Nano didn’t have all this noise.