Over at the Just Jazz Guitar boards, there has been an ongoing thread about "Oscar Moore" tone, and I would chime in from time to time on it. Tim Lerch recently posted about his observations on the effect of the guitar volume knob position as it relates to the tone of the amp. He wrote:

"It is my belief that back in the day the players would almost never have the volume knob on their guitars all the way up, they would often run them quite low in the half way up range. This changed the quality of the sound quite a bit allowing them to get a cleaner slightly brighter tone than would result if they had played on full." (See his specific post, and the rest of the thread HERE)

I had noticed something similar, but I always seemed to need my volume knob all the way up to get sufficient stage volume. After seeing someone else observe it, I did some playing, and I tend to agree completely. 

I decided the best way to show you what I was hearing was to make video demoing different volume knob positions. Enjoy.

And here's a bonus clip of me just playing through a bit of "It's Only a Paper Moon":