Hi Antonio,
Just been doing that today with a new accordion setup with a friend, but I still would love to hear about the pro's.
We have made a 20 sec recording using the mixer, and played it thru on SOLO headphones.
We then used the 6 channel eq, and swept a high Q, high dB to find the frequency (in this case the accordion mic is a capsule inside and was picking up finger fretting the bass keys).
After we found the frequency we reset the 6ch eq, and went to a dynamic eq, turned it on a tuned that freq, set a proper Q and dialed a negative high level filter, then we tried to find a proper low and high threshold.
The key point is finding this point where we find the right thresholds to dim down the offending freq and still allow harmonics to pass when needed.
(For my bad this freq is very near the instrument central frequency).
Is this right as a workflow?
PL