Put some weight in the seat while it's parked. Bags of beans, books, buckets.. ect.
After seeing how the seats are constructed I think you could get them to soften up. When I first got the car, nearly 9 years ago I thought the seats were too firm. I can now tell that the drivers seat is not as firm as the passenger. It's the same foam construction other than the passenger presence sensor, which doesn't effect the seat feel.
Good Luck.
