Buyer Beware - Foam Does NOT 'Loosen Up'

If you like a very firm mattress, this is for you! I bought this mattress because I have a very bad lower back - degenerated lumbar disks and need good support. I'm 5'9", 130 lbs. This mattress has a lot of foam on the top above it's firm coils and was very comfortable in the store. I've had this 2 years now and it's never felt like the one in the store. It's VERY FIRM. When I returned to the store w/complaints that this mattress is much firmer than the 'same' one in the store, I was told that the foam would 'loosen up' over time. In the interim, I should remove all the bedding from the bed and walk all over on it w/hard soled shoes to 'loosen up' the foam. The foam has never loosened up and I'm now seeking my 2nd mattress topper which will be 3-4" of soft foam. I will say, the support is there in this bed and it's got no sags in it, but I'm not very big and what weight I do have is spread over the length of the bed w/my height. My hips and shoulders hurt unbearable w/o any sort of topper.