I don't know if this is your problem, but I found the Jazz seats hurt my lower back because there is a hard lump which presses in the small of my back . This lump is the headrest anti whiplash actuator , and to see if it is giving you a problem try pressing the seat back in this area while pulling the headrest forward- you will feel the lump receding as you move the headrest. The attached photo shows how to adjust the pressure on your back my using cable ties to hold the actuator in a different position. This is a tip I picked up from the Fit forum, and worked for me as I found my seats very painful from this pressure before I modified them . If it doesn't work, you just remove the cable ties.
I did find on my previous car, a Hyundai Getz, similar problems from a sculpted seat , and in that case a Halfords padded seat cover worked , made the seat comfortable, but looked horrible !
I hope this helps.
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