One more point: the difference between instruments is NOT defined only by raw sound. It is a lot to do with what (musically) it allows the player to achieve. That can't easily be measured, and can easily be misunderstood.
Sorry for jumping in on this thread but I just wanted to comment on Nic's remarks.
I can't agree with him enough about what he said in the above snippet. I used to play the classical guitar and the musician is keenly aware of differences in tone and touch which a listener would never hear. And, perhaps more importantly, how the instrument responds to your playing - some instruments seem to come alive and are totally responsive and open up a different world of colours and possibilities. These might seem minor to a listener but IMO they profoundly affect how a musician plays - and that is often very apparent.