I have heard it in phones and equally with monitors. I have also experienced it with cardioid mics, but they need to be closer to the piano.
Maybe there's another explanation or an aggravating factor. How many of you are using mics with transformers?
PSW Recording Forums: Klaus Heyne's Mic Lab => Can you hear the transformer in some of these mics?
I am only using transformerless setups, but still hearing it.