" Musicians play things in certain keys and tempos for a reason" - Atmasphere
That's exactly right, and the point of my earlier comment. Deviation from the musicians' intent has a significant impact on the music. That's not to say that the music can't be enjoyed if there are small deviations, but there is no question that it's full impact can't be realized.
Atmasphere, I agree that speed stability affects sound staging. I also believe it affects perceived detail retrieval, and timbre. I use a straight
tracking arm (ET2), and I still hear improvements in staging when speed stability is good. When it is not, staging is not as defined and is slightly confused.
I think that in our attempt to arrive at a scientific explanation for all the perceived aspects of music playback, we overlook the very real possibility that there are many things that we don't yet fully understand; probably never will. As I said in an earlier post, time and rhythm are where the heart and soul of music lie. The subtleties of rhythm are extremely fine
and can be very elusive for the record/playback process. The relationship
between rhythm and timbre is likewise very important and difficult to understand. For example: experiments have been performed that show that the characteristic (individualistic) recorded tones produced by players on any given instrument are very difficult to tell apart if one removes the initial attack of a note, and listens only to the sound that happens after the attack. Speed instability alters the sound of the initial attack since the initial attack is connected to a point in time. With this in mind, it is not difficult to understand why speed instability can affect just about every aspect of sound.