MB designs their seats to be more taut than the typical foam seats. I think you will find it difficult to weaken the springs. A more common complaint is the weakening of the seat box springs at the driver outboard side in the front.

Every MB I have owned has that problem. MB actually offers a custom foam piece to firm up that side. I know it exists for the W124 model.

I had a '90 300E. I bought a new seat box and pad, which, when installed, firmed up the seat bringing it back to new. Then I traded for my '92 Sportline with the Recaro seating. For the first month I seriously considered selling the interior and replacing with standard W124 seats. Then my body grew accustomed to the firmer seats with stiff side bolstering. I have driven that car over 143K miles mostly very long interstate traveling. Suits me fine.

Maybe you could just buy one of those memory foam car seat cushions you can find on the internet.

Steve