Hi Kimmie,
The density of memory foam (or polyurethane foam) is not related to firmness because any density of foam can be made in a wide range of firmness levels from ultra soft to much firmer. For example … the 7 lb tempurpedic memory foam is generally regarded as “softer” than their regular 5.3 lb memory foam (once they both warm up) even though it is denser. Density is the biggest factor in the durability of a material though so all other things being equal … a 3.75 lb memory foam will not be as durable as a higher density memory foam.
Softening is also not the same thing as body impressions. Body impressions are the last stage of foam degradation and softening will happen before actual impressions form (failure to come back to the original level surface with no weight on the mattress ). How much of an effect any softening had on you would depend on how you interacted with all the mattress layers (not just the memory foam) and where in your “ideal range” of tolerance you were in terms of pressure relief and alignment. For example if there was a soft layer on top of a mattress that was say a couple of inches thick and this was over a much firmer layer … if the softer foam softened and allowed you to “go through” it more … you could feel more of the firmer layer below it and the mattress would feel firmer. If on the other hand there was a thicker layer of softer foam on top (say 3 or 4 inches) then the softening of this layer may still be enough to isolate you from the firmness of the firmer lower support layers but could compromise alignment (let your heavier parts sink in too far relative to other areas of your body). Which way softening would affect you would depend on the layers in the mattress and on your own body type and sleeping style.
Putting a firmer layer on top of this would lessen the regular mechanical compression of the softer memory foam and it would last longer but it would also affect the performance of the memory foam (it would lessen the heat reaching it and while it may last longer … it would also not soften and perform the way it was meant to perform or provide the pressure relief of the mattress without a firm topper). Adding another softer layer on top of softer memory foam would also result in the lower layer lasting longer so that the “feel” may be similar but would add to to the risk of the heavier parts of the body sinking in too deeply and could lead to the misalignment of your spine.
All of this is part of the many “tradeoffs” that are part of choosing a mattress … in this case the tradeoff between a lower density foam and the other advantages of the mattress. If you do go in this direction I would first buy the mattress before deciding on any topper to add to it. Because it is 3" thick (not really thick or really thin) … it may also be inside your “range” enough that either the initial softening of the memory foam (memory foam will soften more rapidly while it is breaking in over the first few weeks or so and then the softening will slow down to a more gradual process) or the ongoing softening will still be in your range over time. If you were “on the edge” of your range in terms of pressure relief or alignment then the softening of the memory foam would put you over the edge more quickly. This is also why personal testing can be more accurate than buying online.
The price for the Shipshape is a good one (I generally use http://www.adjustablebeds.org/ as a pricing reference point for adjustable beds because their online prices are among the best) and the Shipshape itself is one of the better values in the L&P lineup for a basic model compared to other manufacturers (L&P is often overpriced in their higher end models compared to Ergomotion or Reverie).
In almost all cases … an all foam mattress (no innersprings) is suitable for an adjustable bed but in a few cases a manufacturer may not recommend them (usually because they are using a stiffer or lower density foam in the base layers that may not be as durable with constant bending). So while it’s probably fine … I would email Spaldin just to make sure.
The foundation thread here includes a couple or links to inexpensive foundations (actually foundation/frames) that are from WalMart.
Phoenix