Thank you for the interesting and informative analysis. What I find especially fascinating is the boldfaced part about the improved tone with a lower stool. There's probably a thread about this somewhere, but it had never occurred to me before. I assume this would relate to the lower angle allowing the natural force of gravity to bring your arm weight into the string(?)
Lee, I think you're right on point here. With the more horizontal angle I had both better leverage, and had to do less work with the strings. I actually enjoyed the feeling, it was like a sigh of relief. The higher angles forced me to balance more, and by working against gravity forced me to do more work than I think I should. At the extreme high of 36" I felt like I would have been better off just standing and bending.
Once I give my back a rest I'll be trying/retrying 3 stools, the 30", a 26" and the 24", to see if I can get a more accurate read. This is the range Im shooting for