… the piezo pickup in my old acoustic bass guitar which the previous owner implemented the wrong way so that it now sounds like crap…
Yes, sometimes things are so "fundamentally wrong" that the best way to fix it is by taking unconventional measures.
For the above case, apart from the parametric EQ for finetuning, I have used the Multiband Compressor to radically suppress the frequence range that I don't want to be there. It works.