Suteetat:
I also play the piano, and have decades of classical music training. When I sit at a grand piano and play, I hear the sound as: powerful, absolutely. Big, yes. Immediate, yes. Complex, yes. Highly tensioned, also yes. Warm, no.
When I play an upright or a console (short upright), it sounds duller and a little less of everything than a grand, but still I don't know if I could describe the sound as "warm".
Now if I listen to a piano from a typical audience seat, where there is appreciable physical distance between my ears and the piano, or the hall is filled with well-dressed patrons, I could well understand the sound being described as warm (or at least warm-er than the performer's perspective).
However, on the majority of recordings, microphone placement results in a sound that is far closer to the performer's perspective than the audience's (I say this based on being present at various recording sessions and being able to listen to the mike feed, and being able to climb to where the mikes are located and verifying the sound with my own ears).
Switching to audio systems, I don't like to listen to the piano on many warm-sounding systems, because I find the pitch to sound uncomfortable. Nor do I like to listen to complex, heavily orchestrated music on warm-sounding audio systems, because the onset of congestion tends to be much earlier than with more neutral systems, resulting in a sound that I personally find messy, sometimes ugly. FWIW, this is true of digital as well as analog source components.
kind regards, jonathan carr