Eames Soft Pad: high or low back?  

My thoughts (maybe not very helpful)
Perhaps you're asking the wrong question.
My wife and I spent a long time last year looking for desk chairs to replace my Knoll Bulldog and her Series 7 swivel armchair. We really wanted to find mid-century designs that would work for us, but office chair ergonomics have improved considerably since those old chairs were designed. After trying all the classics and a whole bunch of more recent chairs, we had to admit that the creepy-looking HM Embodys were clearly the most comfortable and functional chairs for our needs.
If you're going to spend a substantial amount of time working in your chair and you're concerned about what it'll do to your back, you might want to consider buying a newer design instead.

Yes I total agree. The low...
Yes I total agree. The low back is much more comfortable compare to the high back. I'm 1,89 cm tall and sit / work daily on a Vitra EA217. Furthermore, it also depends on whether it is a new or an old Soft Pad chair (more served).

Agree here too. My father...
Agree here too. My father has an EA219 and I often use an EA217 - Find the low back far more comfortable. Horses for courses possibly as I do know people who prefer the high back, my dad included.

I've never sat in an Embody, ...
I've never sat in an Embody, but I have a Mirra at work which I initially thought was comfortable but later on realized after sitting in it for long hours, my back tends to hurt. On the other hand, on the soft pad low back, that hasn't been an issue.
People always say you should try out a chair before purchasing, but the truth is you can't really sit in a chair for HOURS in a show room to test out how it feels for extended sitting.
So far, I think the soft pad low back is my preference.

The lower Soft Pad is...
The lower Soft Pad is fantastic. A worthy investment. 🙂