I have built custom furniture for over 20 years and this is an easy fix. (I will say Im surprised that Pottery Barn makes anything that is stiff) You can buy rolls of poly at an upholstery supply house. Remove one of the white foam layers on one side and replace with 2 or 3 layers of poly. When you cut the poly square (the size of the cushion) cut the bottom one about 3" smaller than the top ones. Place it in the center foam and then place the larger ones over it. That is called crowning. It will give your bottom cushions a slight puffy area that is higher in the center and makes the whole sofa look more expensive. This whole project won't take more than 2 or 3 hours and will cost under $100. Good luck!
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For the chairs
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