Are you using our ACX-Check tool? This has come up enough times that FlynWill developed a tool to make all the cardinal ACX measurements in one swoop.
http://wiki.audacityteam.org/wiki/Nyquist_Analyze_Plug-ins#ACX_Check
And I wrote the short page on what those numbers mean.
https://forum.audacityteam.org/t/measure-between-23db-and-18db-rms/32770/16
You found a violation of my assurance that any mixer with 48 volt Phantom Power should not have sound problems. You have “The Dreaded Yeti Curse…” When you stop talking, you have that high-pitched mosquito whine in the background. That’s not a simple noise reduction problem. That’s rough to get rid of.
I need to read over your information again.
Koz