Another +1 for what others have indicated. Generally, 1M should be your go-to impedance. As you turn it down it acts much like a tone knob on your guitar bleeding off high end. It also reduces sustain and signal strength.
A couple of the most common uses for lower impedance are when using exceptionally bright pickups, or, when using a fuzz distortion at the beginning of your signal chain. Vintage Fuzz blocks can require a lower impedance to achieve the “correct” tone. Experiment with different lower values and select what sounds good to you. You may then have to adjust the tone and level downstream.
There may be a few other blocks on the QC that respond better to lower impedance if it is anything like the Helix. Don’t know which ones though. Use your ears. Some vintage Wahs are also said to sound “better” with lower impedances.
Anyone aware if there is a listing of per-block impedance recommendations out there for the QC?
See @NigelGBR 's excellent post on the topic as well at: Quad Cortex Input impedance explanation
Ultimately you want everything gain-staged correctly, the ‘Output’ parameter on the amp blocks is particularly useful for this. You can also look at increasing your output level, rather than your input level. Raise it to +4db for example via the I/O swipedown page or by long-pressing the individual output blocks. Whatever you are monitoring through may require more output level.