why 3D Holographic sound

That's happened on my system--something I noticed even back in the 80's with much different gear. I believe the main culprit is sound reflected from the side walls and perhaps the ceiling and floor. It can be remedied to some extent--perhaps a large extent--by adjusting toe-in and tweeter angle. (I think it's 90% a tweeter issue.) Also, if your room can tolerate it, a lot of acoustic treatment can probably make a big difference.

But even when I've got everything just right and have very nice imaging from my listening chair, there seems to be a sweet spot fairly near the speakers and above them (i.e., that I get from a standing position) where the imaging is particularly fine. (The spot that I vividly remember with my Dahlquists in the 80's was only about a foot away.)

Two likely issues. One is the vertical dispersion of the tweeter is different for your standing and sitting positions. You might try changing the tilt of the speakers if possible.

The other is the reflected sounds probably differ between standing and sitting. Look for differences in the first reflection surfaces (left, right, up, down and even behind) between standing and sitting. Strong reflections will muddy the image. Changing speaker position or adding sound absorbing materials at critical points may help.

You'll probably have to experiment to find a solution, but it will probably be centered around a factor noted above.

depends on what you're sitting on. a high-back chair with a cushion by your ear will of course sound very different than a shorter chair with nothing immediate by your ear absorbed or reflected