Just f.y.i.
More and more people seem to be discovering the intelligence of making one's own mattress.
http://danielwebb.us/projects/bed/

That was interesting. I'm always intrigued by people who make their own mattresses. When I was between the latex mattress from SleepEZ that I had to return and the innerspring I'm currently sleeping on, I slept on a mattress very similar to the one in the article, only since it was just temporary, not quite so high-end in the materials department. It consisted of 3" of 35 ILD PU foam--which at my weight is so firm that I hardly make a dent in it-- and my 2" 4lb memory foam topper. I had it on a regular mattress foundation. I found it so hard that I had to insert an old egg crate topper between the two layers. I suppose I could have bought another 2" of 28 ILD PU foam to pad it out a little, but I'd already come to the conclusion that my back really needed the rigid support of an innerspring.

We, too, tried sleeping on a 4" HR foam core - around 45 IFD I think it was - with memory foam on top in 2 and 3 and 4" layers (one layer only, but tried it in different thicknesses and types).
However, I don't think we EVER tried the high-density memory foam (5lb foam). We couldn't find it. The highest density we used was 4lb and that is just too soft to give support, especially in anything above 2". I did try the Tempurpedic mattress, which I think is 5lb foam, but as this guy points out on his page, the problem with the Tempurpedic is that they put the memory foam on too thick.

So I think his idea might even work for us and I might try a slightly different approach by putting a 5lb density 2" memory foam topper on top of our HR foam of various zones and densities.