For my birthday this year, the family got me a book called "Cockpits of the Cold War" where some of the warplanes of last 40 years are reported on by some of the pilots who flew them. One common complaint about the Boeings (B-47, B-52) was how bad the seats were. They are probably using the same design.
I have had back problems for years and it has gotten worse since I've been on the Boeing. A few years ago I was sent to physio by the clinic and the therapist remarked that she had seen quite a few Boeing pilots with lower back pain. She got someone to take her into the simulator and had a look at the seat. Her comment was that we were all setting the seat adjustments wrong. She said to raise the back up as far as we could get it and extend the lumbar support as far as is comfortable. While it's not that comfortable, I have had less back pain since I started taking her advice. I have also used a cylindrical back pad (left it on the plane) and that helped also.