Switching from the 9" to the 12" V arms caused a very different change to the sound. The longer arm sounds smoother and more transparent. There is increased detail and clarity. Small tonal nuance and spacial cues are more evident. Instruments and voices have more texture. There is just much more music and information. I suppose this is in large part a result of decreased distortion and tracking error due to the extra length. Imaging is more specific and sound radiates more naturally from the source, be it a voice or an instrument. Sound fills the space between instruments more completely and creates a more organic whole than the somewhat outlined images of the shorter arm. The sound is less flat and there is a better sense of harmonics. Upper frequencies don't have quite the energy that the 9" arm has. I guess I would say the details are less forced and sound more natural.
Interesting. That's pretty much in line with what I heard when I compared.