I testdrove an ST with my wife and absolutely loved the seats, they supported every bit of my back.
The wife however, not so much... After about 10 minutes of driving she had a back ache (lower part of the back, so probably same spot as you)
Needless to say we didn't buy it.
In the ZS she has no problems though, so if you can get the seats from a ZS installed, that might solve the problem.
Also have you tried raising the back to an almost vertical position ? It might help your upper back "connect" with the seat.