Looking for a super soft mattress

Hello please can you help, I am passionate about getting my next mattress right. We bought a John Lewis 12000 Natural Collection Firm mattress (Harrisons) about two years ago. My husband is 13 stone and I weigh 8.5 stone and at no time we're we advised about our weight to firmness ratio. Obviously when the bed arrived it was like sleeping on a plank of wood with bad backs and hip ache, luckily John Lewis changed it 18 months later as being mis sold, we opted for a Vi-Spring Melford Supreme as we were reluctant to go back to the same mattress in a softer tension, however, maybe the internal upholstery would of been more suitable. I went for soft and my husbands side is medium, however, both sides feel the same and still quite firm, I am constantly testing it to see if I can tell the difference but I can't. I am a side sleeper and so is my husband, I find now that I have shoulder pain. I didn't realise that I needed to be looking at the internal upholstery weights as per your website and I am very disappointed. We now need a new Super King mattress this summer and my daughter is taking our Vi-Spring as we will be buying a new bedstead. I am looking for a super soft sinking feeling on my side and my husband prefers medium.