A very important point so rarely discussed, and you are sage to bring it up. Perhaps posture has more to do with fine tea than the water or pots even. In meditation, posture is everything: It affects the way our Qi moves, affects our breathing and blood flow, and amongst many other things, even our integrity. Dignity doesn't slouch. In some Zen sects, the meditation posture itself is enlightenment. Actually, we must practice in all postures, but the upright one is especially useful, as slouching is invariably associated with some form of inner turmoil. Try moving your tea to the floor, or at least sitting upright between the Earth and Sky as you pour your tea. imagine yourself rooted; imagine a string from your head to the sky, balancing you perfectly here. What is the difference?
Later, you may find that not only the straightness of your back, but also the angle of your arms affects the flow of Qi. As in Taichi, for example, the arms and/or elbows should never be parallel or higher than the shoulder. Try raising and lowering your elbow (like a bird's wing) and find the point at which the pinch occurs. Try using your kettle to pour two cups of water, one below and one above said pinch. Are they different?
Thank you for the insight.