Like JMS said, there is no "absolute" sound of an instrument except for the insturment itself in a real space. That sound in it's entireity is almost impossible to caputre on a recording, but having more channels helps to give more spatial information to the illusion.

I've tried to hammer this home too much I think, but the fact is that Diana's piano would sound just as nice if it was recorded on a cassette deck with 5.1 channels (except for a bit more hiss) - the recording medium is almost irrevalent. It is 99.999% in how the mics are set up and how it all interacts with the recording room. Recording something like a big piano is a huge feat and there are 1000s of variables on how to approach it, from sticking the mics inside the piano to a stereo crossed pair of bi-directional mics. Then there is the issue of how many channels are used - stereo or 5.1. All this influences the sound way more than the recording medium.