It's amazing that you are able to answer questions like this, thanks!
I'm trying desperately to get back to playing piano without pain. I strained my back recently, as I do every few years despite good core strength. I've got your book, and am trying to implement stacksitting. Although I feel like I'm sitting pretty well (see photo), I still get sore (lower back pain) after even a few 10 minute sessions at the piano. I need to be able to play at three-hour gigs, and practice for several hours per day. Perhaps I'm not waiting long enough for my recent strain to heal. The photos below show me stacksitting, stretchsitting (which doesn't seem completely practical for piano), and playing a year ago.
Any comments you have would be appreciated. Thank you.