For me (having sold pianos for a few years) "piano" tone means that everything comes out ....the full spectrum of tone. There is no "bump" in the lows, midst or highs. You get it all. It's not necessarily a good thing for me as I prefer warmer tones. Particularly grand pianos have loooong strings in the bass register and huge sound board. If the piano is made well it doesn't "favor" a particular frequency range, rather allows all frequencies to leap off the soundboard equally. This can be affected by voicing the hammers somewhat. European pianos tend to be voiced more mellow and Japanese more bright. This is accomished by harder it softer tips on the hammers and a few other factors (think playing with fingers as opposed to a pick).
Shirt version: Piano-like tone from a bass to me is akin to "uncolored" tones that many strive for with amps and cabs. One player's "clarity" is another player's "sterile", but you get the drift.
