Hi cari7440,
There really is no “best density” except to say that higher density will maintain the original properties of a foam for longer than lower density if all the other durability factors are equal (see post #4 here for more about the relative dirability of different materials and designs). Defining “best” always depends on the specific use of a material and on the many other factors involved and on the budget range you are working with. Best is always relative to all the tradeoffs that are most important to you and in a lower budget mattress may include lower quality foams than “best” in a higher budget range when what you are paying is part of what is important to you.
If the latex in your mattress is right on top (except for the quilting layer) … then this would be good news and it would increase the durability of the foam layers below it. Layers that are closer to the surface are more stressed with compression and this is where the durability of a layer is the most important. Thicker layers of latex would improve it even more. Once you are down into 1.5 lb polyfoam though … you are on the top end of the lower quality polyfoam range and 1.8 lb is at the bottom of the higher quality polyfoam range.
Phoenix